@maxipad640:
I would admit that, yes, you could probably bit-bang I2C & SPI interfaces as part of your senior design project. However, doing so would require a great deal of in-depth detailed hardware knowledge & very likely, many hours spent poring over hardware manuals underlying the Teensy family of products. I would argue that, in current project activities, hardly anyone "re-invents the wheel" like this, with exception of those folks who desire to better understand what goes on under the covers, purely for their own educational purposes.
Using existing libraries has many advantages, some of which are as follows: a) someone else has already spent the time poring over the hardware manuals in order to fully understand the necessary implementation of the library, b) someone else has provided tested, working (very likely bug-free) support thru these libraries, c) many other users have used these libraries, standing up a large base of verified operations making use of these libraries.
if, instead, you were to implement your own replacement(s) for these libraries, even with your prior STM32 experience as you mentioned, your success is heavily dependent upon your correct interpretation & understanding of the hardware manuals. I would predict that your implementation is much more likely to suffer from unknown & difficult to troubleshoot software bugs. I don't know what else you have to accomplish on your project, but a 2-month schedule leaves hardly any time for wrestling with non-working code. Your ultimate success is much more likely if you are allowed to use existing library support.
I would suggest that you politely & diplomatically inquire with your instructor as to the intention(s) behind his request that you "roll your own". If it is as simple as "you need to fully understand how things work", my humble opinion is that your time would be far better spent studying the existing library capabilities, & then writing a detailed analysis of how it accomplishes its intended purpose.
Hope that helps & best of luck on your project . . .
Mark J Culross
KD5RXT