VPNs and the law · Plain-English answer
Yes, schools can usually detect VPN use on their networks. Firewalls recognise VPN protocols and known server addresses, even though they cannot see what you do inside the tunnel. Many schools block VPN traffic outright, and using one may breach the acceptable use policy you agreed to.
School and university networks typically run filtering firewalls that inspect traffic patterns. VPN protocols have recognisable signatures, and the IP addresses of commercial VPN servers are published in databases that filters subscribe to. So while the network cannot read your tunnelled traffic, it can often tell that a tunnel exists and either log it or block the connection attempt entirely.
On school-owned devices the picture is stricter still, since management software on the device can see activity before any VPN encrypts it.
Using a VPN is not against the law in the UK and most countries, but school networks are private, and connecting to them means accepting an acceptable use policy. Bypassing filters usually breaches that policy, and consequences are disciplinary, such as losing network access, rather than legal. Schools also filter for safeguarding reasons and may have legal duties to do so, which is why the rules exist.
On your own device using your own mobile data, none of this applies. The school controls its network, not your phone contract.
ROOT VPN is free to start with unlimited data, no card needed, and no activity logs.
Get ROOT VPN, free