The PEC divide up the levels. Is it possible to ensure that a PEC only sees the classes he/she is responsible for?

Yes, this precision is carried out in the display Resources > Classes >  List, by double-clicking in the column PEC.

By ticking the corresponding option in the menu My preferences of their Client, the referent PEC will only be able to display information for the classes for which he/she is responsible. When adding recipients to a discussion, information, etc., the recipient is also pre-selected if the assigned class or one of its students is involved: for example, in a notification distributed following the programming of a punishment.