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Schools are required to allow students to be released for religious programs. This does not mean schools do not have the ability to create policies that protect the school's instruction time.
Read more about "May" and "Shall" state laws

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§ 11.21. Religious holidays and religious instruction.
(a) Upon written parental request, and in accordance with the policies of the district’s board of school directors, students may be excused from school for religious holidays observed by bona fide religious groups.

(b) Upon written parental request, a student shall be excused from school to attend classes for religious instruction under section 1546 of the Public School Code of 1949 (24 P. S. § 15-1546). The excusal shall be limited to a total of not more than 36 hours per school year.

(c) A student’s absence from school for religious holidays or for religious instruction shall be recorded as an excused absence. A penalty may not be attached to an absence for religious holidays or instruction.

Authority
The provisions of this § 11.21 amended under section 1317(a) of The Administrative Code of 1929 (71 P. S. § 367(a) (Repealed)); and sections 1327, 1330, 1372, 1511 and 2603-B of the Public School Code of 1949 (24 P. S. § § 13-1327, 13-1330, 13-1372, 15-1511 and 26-2603-B).

Source
The provisions of this § 11.21 amended December 19, 1986, effective December 20, 1986, 16 Pa.B. 4874; amended October 22, 2004, effective October 23, 2004, 34 Pa.B. 5798. Immediately preceding text appears at serial pages (244457) to (244458).

All school districts, regardless of whether they are required by law to allow released time, should implement good policy and be prepared for when a released time program may approach the district. The time to set the policy is before an active program is pressuring the district. Research polices the district can implement to protect teachers' and students' instruction time. Set polices that limit disruptions to students and the ability for proselytization to occur, impacting other students of faith or no faith. A more detailed write-up on school board policies can be read here.