Don't bother with a digit mic.

Get a plain jane vernier, such as this one for simplicity and cost or go to a fully digital one like this for flexibility and speed.

As a student, you probably don't need it to come with a certificate of calibration or traceability which sometimes costs you quite a bit extra. You really need to get practice checking your tools using gage blocks or a standard, a cert just states that it was in compliance when they tested it after making it, not that it is in compliance when it's in your hands after a few months of class.

Speaking of trust, you should inherently not trust any of your tools and verify that they are good to a known standard regularly. Digitals are easier to re-zero a few tenths (or many thou) off true zero, but that's not to say some ham-fisted fellow student couldn't bung up your regular mechanical mic either.