Is it going too far to have a (tasteful) penis in my artwork for school?

Noway.

Art is art.

Express yourself and your vision.

This is what art is for.

DO IT. Geez.

As long as it's done in a tasteful way your teacher should have no problem with it. The nude figure is so important for a young artist to master because it provides the basis for proportion and rendering. Your teacher should commend you for going beyond the requirements to learn a skill that will stay with you and be extremely beneficial.

The teacher might be unable to display the work in school but ask if you can submit it in a local show instead.

Good luck!

If you're drawing a nude figure, then it is fine - one should define the sex of ones subject. Have a look at some Greek/Hellenistic art for ideas.