Also: whole schools went to the demonstrations. Pupils and teachers (because they don't like to work in marode buildings either). And we cannot claim that all the pupils and all of the teachers of a school are communist.
But yes, the communist party in France has about 43000 members. But we nation has 66.7 million people. So we shouldn't give this few radicals more influence than they deserve. If anything, the populist right is way more influential and numerous.
Somehow in France the demonstrations have a way higher likelihood to get violent than e.g. in Germany. Just think of the yellow vest demonstrations. Also, the french police is known to be violent, too. Perhaps they don't follow the de-escalation strategies the german police often (not always!) follow? In France, demonstrators lost a hand, teeth, an eye due to police force. That's certainly not as bad as the 9220 people killed by US police in 2024. But for Europe, it's a quite stark expression of government violence.
We must also understand that the majority of journalists are like the Demonstrators from the Harry Potter universe. They life of emotions. Often they artificially create them. I mean, why show a picture of 20000 people just holding up posters --- in a language you won't often be able to read --- if I can show a picture of 4 people torching down a car?
Just so you have a clear image of what is going on, you have pseudo student demonstrating asking for better infrastructure that are burning down their schools. Even brand new support modern schools...