4-15 agreement has some consense on this issue: the school which accepts an applicant after 4-15 must get the written agreement that the school the applicant has accepted its admission agrees the applicant not to attend its program.
If you cannot attend the class, do please advise your school of your final decision, so that the school can make the place available for others. Though troublesome, your school will thank you for your professional way of handling this issue properly.