as salaamu alaikum,
i am currently a medical student. i also have a business on the side which alhumdolillah generates some income. but there are times in the year when i have maintained my nisab for a year but have not payed for my upcoming semesters tuition because the school has not sent out the bill for the tuition yet. what should i do? is the money i have saved due for zakat even though i know i have bills which will exceed the savings which are coming up?

All praise be to Allah, and may His peace and blessings be on the greatest messenger, Muhammad.
If the bill for the future semesters has not been generated and you do have the nisab, you must pay the zakat.
Allah knows best.