I understand your reluctance to tell your parents or teachers for fear that this turns into a paperwork nightmare that involves all your teachers. Were I in your situation, I might try two things before reevaluating the situation:

1. Tell the teacher that it is really hard for you to concentrate in the lab and ask to do it before/after school or in an alternate setting (for example, could you do this in a classroom when it's unused).

2. Talk to the guidance counselor, if you trust that person at all. Again, I would try the concentration excuse first. If that doesn't work, you might broach the ptsd subject. It may be that the teacher and guidance counselor will be willing to help you work around the lab issue without drawing up all the 504 paperwork.

And though I know you won't like it, you might consider:
3. Tell your parents what's going on and discuss the possibility of a 504. They are put into place in order to give students the accommodations they need. They are not intended to humiliate you. Teachers are bound by confidentiality (if they don't, they are violating the law).

PTSD is a disorder that might always be with you in some form. Needing an accommodation to feel more comfortable in a school activity does not mean you are sick, unable to be independent, or weird. It means you are jumpy because you were traumatized. That's it.

I wish you luck.