I will be 16 when i go + i have taken my gear to England before inc gun but without my gas tank. Coming from spain.
At 16 you should not be owning / possessing a paintball gun in your own rights, but everyone does so that ought not be an issue unless you get into a situation that leads to it becoming an issue.
Not transporting a cylinder will be easier, and if you can accept 3000psi then buying a UK cylinder will make things simpler
(Just for info - what is the Spanish requirement for cylinder standards? e.g. DOT is USA spec, Pi is UK spec. Do Spain recognise Pi spec?)
You could go with covering up your paintball activity, but this risks ultimately getting caught.
Unless you can make other arrangements to have someone store it for you and be your transport etc then its going to be difficult without getting the school on board.
Until you know where you are going you can't get much further on how acceptable it will be.
Talfryns method of getting parent support for you being part of a team and participating in tournaments etc should help with the matter, assuming this is your chosen flavour of paintball.
Ultimately the school can have rules but they ought not be able to prevent you taking part in activities of your choice. So you ought to be able to play other paintball such as scenario if you wanted.
With your parents being in Spain that would most likely be the issue that the school would have with letting a 16 year old go off on his own to play in the woods