It should not be imposible to enter a class room and arrest somewhere there in general. But there are actual standards and also issues of common sense usually agreed to in France (and a lot of other countries).

You don't run into a class in session to arrest someone there, if you can just get the person called out of class and do it separated from everyone else. Just like you for example don't focefully arrest someone or initiate a chase in crowded areas, if the case isn't justifying the risk to bystanders for the reward of making the arrest now rather than later. That's okay if there is an actual risk right now that justifies making the arrest now and making is fast. It's not okay to do this in a crowd just as a show of force when it could be handled much easier and safer.

That's just common sense of minimizing the potential for any kind of escalation.

So yeah, this has nothing to do with the fact that you should not be allowed to arrest someone in his classroom. It's about general standards in arresting someone, that were violated purely to make a political point of showing their hard reaction.

If that guy has a weapon and is threatening someone, sure. Go in, and go in hard. If the guy has made some general threats and is right now just sitting in his regular lesson, call him out of class under some pretense and handle it professionally instead of making a show out of arresting him in front of the class. That's unprofessional bullshit made for political reasons.