The German parliament has rubberstamped a new model for military service that aims to boost its armed forces, as thousands of school pupils demonstrated across the country against the plans.

The change will include the obligatory screening of all 18-year-old men to gauge their suitability to serve in the military from 1 January, but does not include conscription, as favoured by some conservative politicians.

If the new model fails to pull in enough recruits, parliament will be compelled to discuss the reintroduction of conscription, the defence minister, Boris Pistorius, told the Bundestag.

Pupils missed classes to take part in climate protest-style “school strike” demonstrations against the legislation in 90 towns and cities, despite education authorities’ warnings the abstention could affect their end of year grades.

“I’m striking against conscription and in opposition to the rearmament that’s taking place, not least as I don’t think the government is doing enough to secure peace through diplomatic means,” said Alicia, a 17-year-old taking part in a demonstration in Kreuzberg, Berlin.

MPs addressed pupils’ concerns that young people’s futures were being put at risk amid pressure on anyone born from 2008 to join up.

Siemtje Möller, of the SPD, junior partners in coalition with chancellor Friedrich Merz’s conservatives, pushed back against what she referred to as the “pure populist” message of the protest. “We are neither deciding today that you will be obliged to serve in the armed forces, nor that we will be drawing lots to send you to Ukraine as cannon fodder,” she said. “That is pure populism, or simply nonsense.”

She expressed the sentiment building over recent weeks that a surge in interest in the German armed forces will ensure enough volunteers will be found to boost numbers to a total of 460,000, consisting of 260,000 active soldiers and 200,000 reservists.

Currently Germany has 182,000 active soldiers and just under 50,000 reservists.

In 2011, under the government of Angela Merkel, Germany suspended its military conscription programme, which had been in place from 1956, in order to modernise it for the post-cold-war world, where it was thought the focus would be on foreign missions requiring the skills of a professional army rather than conscripts needed to fight a war.

Pistorius called the law “a critical step towards our ability to defend ourselves”, adding: “Our allies are looking to us.”

The law was passed by 323 votes to 272, with one abstention. Among those against it were the far-right populist AfD and the far-left Die Linke.

Desiree Backer, of Die Linke, urged young people to stand up against the law, which she said was “anything but voluntary”, as 18-year-olds would be obliged to fill out a questionnaire. She also pointed to evasive answers given by ministers when questioned over whether they would be prepared to see their own children conscripted.

“Young people have other plans than risking their lives for the rich,” she said.

For the AfD, Rüdiger Lucassen criticised as “superficial” attempts to attract young people to military service by increasing their pay substantially and boosting fringe benefits, such as the financing of driving licences and language classes.

“Soldiers who come for the pay have no solid foundation for their service,” he said, calling for a military service with a nationalist bent whereby young people would be driven by patriotism, not money, to serve their country. “The German soldier must know what he is fighting for,” he said, arguing that soldiers should follow the centuries-old tradition to “fight for their country, not for a government”.

Critics called his stance problematic for failing to refer in that context to the atrocities committed by German soldiers during the Nazi era.

The historic vote took place ahead of another key piece of legislation regarding the German state pension, also expected to have a significant influence on the lives of young Germans.

Merz had faced an unusual rebellion within his own party from 18 young MPs who argued that the law, which will keep state pensions at 48% of average wages until 2031, would come at the expense of young people who would bear the brunt of demographic changes that meant ever fewer workers were responsible for the pensions of their elders.

The nail-biting vote was considered to be potentially so tight that sick MPs and those with babies were persuaded to be present.

In the end, the rebels were won round by a promise that a commission would draw up proposals for more far-reaching changes to the pension system from next year. The vote passed by 319 in favour to 225 against, with 53 abstentions.

The law also includes tax incentives for people to continue in the workplace beyond retirement to cope with a chronic labour shortage.