Schools can't (or are not supposed to) force you to decide before 15 April.
While the below resolution is for financial aid, most schools honor it for admissions as well.
https://cgsnet.org/ckfinder/userfiles/f ... ug2018.pdfAcceptance of an offer of financial support* (such as a graduate scholarship, fellowship, traineeship, or assistantship) for the next academic year by a prospective or enrolled graduate student completes an agreement that both student and graduate school expect to honor. In that context, the conditions affecting such offers and their acceptance must be defined carefully and understood by all parties.
Students are under no obligation to respond to offers of financial support prior to April 15; earlier deadlines for acceptance of such offers violate the intent of this Resolution. In those instances in which a student accepts an offer before April 15 and subsequently desires to withdraw that acceptance, the student may submit in writing a resignation of the appointment at any time through April 15. However, an acceptance given or left in force after April 15 commits the student not to accept another offer without first obtaining a written release from the institution to which a commitment has been made. Similarly, an offer by an institution after April 15 is conditional on presentation by the student of the written release from any previously accepted offer. It is further agreed by the institutions and organizations subscribing to the above Resolution that a copy of this Resolution or a link to the URL should accompany every scholarship, fellowship, traineeship, and assistantship offer.
CGS has prepared a shareable PDF of the Resolution, which includes a list of CGS member institutions that have expressed support for the agreement. We have also prepared a sample letter you could send to your school deans, graduate program directors, and administrators.