You can put "V.C." next to your name, which is short for the Latin "Vi Coactus" meaning "having been forced".
As a minor this kind of thing is largely meaningless anyway, as minors cannot usually create legally binding contracts. Even if they could, for a contract to be legal it must be freely entered and negotiable by both parties - forced signing invalidates it.
Of course school does not necessarily operate under legal principals. Abusive teachers can punish you on a whim and there is little you can do it about.