Who owns the intellectual property of the website that the programmer developed for the school, and is it permissible for him to sell it to other schools?

The website you were commissioned to create belongs to the school, and you do not have the right to dispose of it without their permission. However, the program and its underlying concept are exclusively yours. You have intellectual property rights over it and the right to dispose of it and create other websites based on its concept. The Islamic Fiqh Academy has affirmed that intellectual rights, such as authorship, invention, and innovation, are private rights belonging to their owners and have a recognized financial value according to Sharia. It is not permissible to infringe upon them.