Does the school you go to need to know that you have the Signal app installed on your phone, checking with the Signal servers in the background for new messages? Even if you chose to use a VPN to tunnel your traffic entirely, is there no other option but for your employer to witness you connected to a foreign VPN server? If you connected to a point at your home, even that could be interpreted that you have something hide.
You could have two phones with different sets of apps in your pocket (one for “business” and one for everything else), but you if you don’t want that you have to ask:
Is there a firewall for Android that can block your usual traffic from leaving the device, by turning on a specific profile based on something like the Wi-Fi name? There are quite a few traffic blockers, such as RethinkDNS, Netguard, or personalDNSfilter, but they assume you want to block the same set of traffic regardless of time and place.
For the record, in almost all versions of Android you can install apps in a isolated environment through Shelter. Apps in this environment get icons in your home screen just like the rest, but don’t share the vpn/firewall connection that you might have active through RethinkDNS, among other things.
You effectively have two sets of apps with different firewall settings. And if you figure out a way to automate the locking down of RethinkDNS (through something like Tasker or Schröder’s Automation,) you would effectively have a mechanism that only lets the small number of apps in the isolated environment work while connected to an untrusted network.