The men stormed the school yard in Gelsenkirchen on Monday afternoon, WAZ newspaper reports, and started threatening people with the fake pistols.
When a teacher confronted the men about what they were doing there they started hurling "disgusting insults" at her and one of the gang aimed one of the blank-firing guns at her, according to the school's headmaster, Berti Oberholz.
The teacher, who was unable to identify that the pistol was harmless, withdrew into the school and called the police.
The headmaster described the young men's behaviour as "disgusting horseplay – especially in light of what has just happened in Paris".
The police responded by sending a large number of heavily armed officers to the scene. Witnesses saw some carrying MP5 submachine guns.
Several of the men were arrested but police were reluctant to give further details.
"But we don't believe there was serious motivation behind the incident. No one was injured," said a police spokesperson.
None of the pistols in the men's possession were dangerous, but were rather blank guns. What the group were doing in the school yard is still unclear.
False bomb alarms
Monday was a day of nervousness and tension elsewhere in Germany due to the terrorist attacks in Paris on Friday that killed 129 people.
In Berlin, police were called out on eight different occasions after people reported finding suspicious packages, reports Berlin newspaper BZ.
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On each occasion explosives specialists were sent to inspect the packages, but were able to give the all clear shortly afterwards – each of the suspicious objects turned out to be harmless.
"We are always grateful when people report suspicious objects to us and we follow up every report. But people should also take better care of their possessions," a police spokesperson said.
The most significant police operation took place near Alexanderplatz, one of Berlin's most iconic tourist spots, where a street had to be closed down after an unattended suitcase was found on a bus.
In Lübeck in northwest Germany, police were also called out to inspect a suspicious package, but again it proved to be harmless, reports HL-live.