By showing a lack of respect for this young girl's religious beliefs, the school risks alienating this young person and other devout Muslims. This makes her more at risk of seeking efuge in an ISIS "caliphate" where she may be fooled into believing that she can truly be a Muslim by the ISIS propaganda experts that go online to recruit vulnerable young people to their cause.
Terrorist have the mentality of leaning towards violence even with slightest adversity. Banning niqab is like challenging their religious practices. This will be a big trigger for them to get violent and protest in the ways they think are right. Their way always involves violence and destruction even if it does not involve people who actually banned niqab. They don't care who are affected. They just want a reason to spread hatred and cause more terrorism.
German court banning a Muslim student from wearing niqab face veil in school gives Islamic terrorists more fuel for recruitment. While the German court is alienating Muslim students and banning their traditions and calling attention to them as different, the students will look to fit in elsewhere. Islamic terrorists would look for alienated Muslim students who have been forced to go without their traditional niqab face veils and hope to use their resentment and alienation from the German court to recruit.
There should be way to balance religious freedoms with dress codes. If the clothing bans are creating more unrest and a sense of disenfranchisement, that sounds like exactly the kinds of attitudes terrorists will prey on in recruiting members. If the dress is a matter of choice for students and doesn't interfere with learning or infringe on the rights of others, why risk it?