Is it considered forbidden for an employee to refuse to share the incentive with his substitute colleague, who shares the burdens with him, after they had been sharing it, starting from the next fiscal year?

The matter in this regard refers back to the regulations governing work in your school or administration. If none exist, then if the administration is the one hiring the person, the agreement regarding deductions and incentives is made with them. If you are the one hiring, the matter returns to what you both agree upon, and you have the right to cancel the agreement unless it is specified by a time, either explicitly or by custom. In this case, you do not have the right to cancel it except with the other party's consent.