Is it permissible to obtain a medical certificate without actually being ill, in order to avoid the mixed-gender sports class at school?

Lying to obtain a medical certificate is impermissible, and it is not permissible for a woman to exercise in a mixed-gender environment where men can see her. One must refrain from this and clarify its impermissibility to the administration, informing the guardian to intervene. If it is impossible to avoid playing or lying, then there is no sin in obtaining a medical certificate to avert harm. This falls under the category of an excuse permitted by Islamic law for a praiseworthy purpose, which is to avoid women playing in front of men and to avoid failing.