Maybe. Some parts of the design may be copyrighted. I think the programming language cannot be, and patents, if any, have probably expired by now.
The problem then is, once you have the clone, you need to get the professors to know about it, and not requiring any specific calculator.
In any case, I'm not sure it would change much in practice. When I was at school, teachers didn't require any particular calculator. Half my class chose Casio, the other Ti. Two competitors are apparently not enough to drive prices down.
