What is the ruling on being absent from school by obtaining sick certificates to exceed the attendance limit, and what is required of one who has done so after repenting?

If you obtained falsified medical certificates, this is forbidden deception, based on the Prophet's (peace be upon him upon him) saying: "Whoever deceives is not one of us." You must repent from the falsification, and there is no blame upon you in continuing your university studies as long as you successfully completed high school.