Last night at the left-leaning London School of Economics (LSE), scores of students gathered to watch election results come in 鈥� and, most hoped, to see Prime Minister Boris Johnson鈥檚 Conservative Party denied a majority. Some here had volunteered for opposition parties or worked on voter registration; virtually all hoped to derail Mr. Johnson鈥檚 drive to achieve Brexit.

So when an exit poll projected onstage indicated a Conservative landslide, groans, jeers, and cries echoed across the auditorium. As the evening went on, the projections were borne out: Mr. Johnson earned an overwhelming mandate for Brexit, while the main opposition party, Labour, suffered its worst result in decades.

鈥淭his result is incredibly upsetting,鈥� says Ella Holmes, a politics undergraduate student. 鈥淲e have been excited to have a voice in this election and, perhaps naively, thought we鈥檇 have more of a decisive [anti-Brexit] swing than we did.鈥�

Mr. Johnson鈥檚 commanding election victory Thursday dealt a decisive blow to a fraught, often quixotic, rearguard campaign to stop Brexit in its tracks. Young people have been a key part of that campaign, including those who were too young to vote in the 2016 referendum and saw an opening to reverse a political project that they saw as crimping their future opportunities and changing the country they will inherit into something unfamiliar. Thursday鈥檚 election represented their last best chance 鈥� and Mr. Johnson won it hands down.

Now, 鈥渞emain鈥�-minded youth have little choice but to come to terms with the fact that Brexit will happen. Far from fighting on, some may be ready to pivot toward shaping a post-European Union future and bridging divisions at home and with the rest of Europe.

鈥淩emain鈥� on the rocks

Thursday鈥檚 vote was an imperfect proxy for Brexit. The Conservatives and the Brexit Party polled less than half of the popular vote. More than half went to parties that either vowed to remain in the EU or, in Labour鈥檚 case, promised a revised EU deal and another referendum. Attempts to forge an anti-Brexit tactical voting alliance came to little as Labour refused to join.

Under the first-past-the-post system, that translated into 365 seats for the Conservatives in the 650-member Parliament, the biggest Tory majority (80 seats) since Margaret Thatcher鈥檚 in 1987. Labour鈥檚 tally was its lowest since 1935.

One problem for the 鈥渞emain鈥� side of the Brexit debate was geographical: Outside Scotland, 鈥渞emain鈥� voters cluster in cities, which have more younger and educated voters who skew left. Analysts say the best predictor of votes on Brexit and other issues is age, not social class.

鈥淚t鈥檚 Trump in the States. It鈥檚 Brexit here. Never in my life have I seen such a divide in age,鈥� says Simon Pia, a journalist and author who lectures at Napier University in Edinburgh.

Emily Douglas, a geography student at LSE, said she hadn鈥檛 volunteered in this campaign but stayed engaged in political debate with friends and family, making the case for Labour and for staying in the EU. She was 16 when the referendum was held. 鈥淏rexit was a turning point. I realized that I needed to get involved and to have my own opinion,鈥� she says.

For young people drawn into politics by Brexit, the battleground may now shift to the United Kingdom鈥檚 future relationship with Europe. Some students say they want to work to defuse a rise in xenophobia they blame on Brexit and its focus on curbing migration to the U.K.

But as the dream of stopping Brexit dies, many of the social and political movements that have led the fight 鈥� and inspired young activists 鈥� are in disarray. A well-funded campaign group for a second referendum, or People鈥檚 Vote, collapsed last month. The Labour Party faces an uphill struggle to rebuild and rebrand after party leader Jeremy Corbyn announced he would step down by early next year. The Liberal Democrats, which adopted the strongest anti-Brexit stance, failed to win seats; its leader, Jo Swinson, even lost her own parliamentary seat. (Rebel Conservatives purged by Mr. Johnson who stood as independents also lost.)

鈥淭he organizational expressions for young people [opposed to Brexit] are all in crisis,鈥� says Stephen Fielding, a professor of political history at the University of Nottingham.

Some will call for 鈥渞esistance鈥� and try to replicate the People鈥檚 Vote movement, but they won鈥檛 get traction since Mr. Johnson has little fear of losing votes, he says. 鈥漎ou can have as many [demonstrations] as you want, but there鈥檚 nothing that they can do to influence Parliament.鈥�

鈥淭here are still things to campaign for鈥�

Election defeats are bitter, but for young activists there鈥檚 always the next vote. Brexit鈥檚 finality 鈥� the unwinding of four decades of fitful integration with Europe 鈥� feels different, but its end destination is still in play, says Tony Travers, a professor of government at LSE who hosted Thursday鈥檚 election party.

鈥淭hey can put their energies into trying to shape the future policy of the U.K. towards the EU, towards a softer, more rational version of Brexit than a harder no-deal version,鈥� he says, referring to the trade accord that Mr. Johnson now has to negotiate with EU leaders. 鈥淭here are still things for people who didn鈥檛 like Brexit to campaign for.鈥�

Ms. Holmes, whose mother is Dutch, accepts that the U.K. is leaving, but is hopeful that it won鈥檛 fan 鈥渓ittle England鈥� nativism that she has seen rising since 2016. She says her generation must push for greater tolerance and inclusion, including toward Britain鈥檚 closest neighbors.

鈥淵oung people should be actively engaging in policies that will keep us as close to Europe as possible. Obviously for many of us, that would be remaining a member of the European Union. But that鈥檚 not possible.鈥�

Tolerance cuts both ways, says Sophie Collins, an LSE graduate who now works at the university. 鈥淚 think it鈥檚 important for people who are 鈥榬emainers鈥� to understand some of the strong arguments for 鈥榣eave鈥� as well. There needs to be more of an active dialogue now.鈥�

She says her parents voted 鈥渞emain鈥� after consulting her and her siblings about their views since it would affect their future. (In other families, the generational divide has become a touchy topic around the dinner table.) And while Ms. Collins was unhappy about Mr. Johnson鈥檚 victory, she also saw a Brexit rollback as improbable, if not impossible. 鈥淚 think it鈥檚 important to realize that a lot of people do want [Brexit],鈥� she says.

Vindication for Conservatives

Amid the groans of students inside the auditorium, Samuel Joynson let out a cheer. He wore a jacket with a blue rosette, the emblem of the Conservatives, a rare species on campus. Mr. Joynson, an LSE graduate who recently earned a business master鈥檚 in France and will soon start a job in London as a business consultant, says the election was a vindication for his party.

鈥淚鈥檝e campaigned to get Brexit done and then we can move the country along,鈥� he says.

Mr. Joynson, who voted 鈥渞emain鈥� in 2016, says youth participation in politics on all sides is positive, but he cautions that students in London may not see their political silos. When he goes back to his hometown near England鈥檚 southern coast, Brexit has a patriotic hue.

鈥淚t鈥檚 more about British values and traditions, and about opening Britain up to the wider world, not just European Union,鈥� he says.

Earlier in the night, Jack Watkins-Hughes, a real estate agent in his 20s, cast his ballot for Labour at a polling station inside a library. He grew up in rural Wales and is dismissive of his village for voting for Brexit. 鈥淚 don鈥檛 want to leave. I鈥檒l still be fighting. But what more can you do?鈥�