Welcome to the US where there are always checks and balances. In the US, adults cannot sign away rights of minors. For example I know of a pre-nuptial that was later updated to specifically state that if the woman wished to have a child, the father would not be responsible for child support if the marriage ended. It did and the court ruled and was upheld that the child support was for the child, not the mother, and she had no standing to negotiate away the child's rights. The father sends the checks every month.
The courts have an obligation to review these matters when they are brought before the court. As a matter of law, and I4E who is a lawyer might comment, that a contract is not valid if any portion of the contract is against the law. If the school district's policy is ruled illegal, they will not have a choice but to re-admit the student. s***ty situation, it reminds me of the kid with the plastic butter knife and the girl who brought Midol from home. Both were expelled within these Zero Tolerance rules for weapons and drugs.