I would treat them like a dream/daydream and bring my thoughts back to the lesson and paying attention. If you have a therapist, I would mention them and work on them in therapy. I remember in group therapy I kept thinking to do a somersault from my sitting position off the couch I still don't know where that came from, 40 years later but imagine I felt unseen or like I couldn't make things happen, was doing a "look at me!" to myself, trying to get my own attention. I did figure out a 20+ year persistent thought, "Guess what we did in French today?" (we never "did" anything :-) as wanting a "normal" conversation/interaction with my stepmother and not being able to figure out how to initiate it. When I worked many years on my relationship with my stepmother in therapy, the thought went away.

The thought/feeling is a symptom of something else, a message of some sort to you, not about throwing up probably but about fear or disgust or anxiety or, if it is only in one subject, maybe something about that subject or the teacher, etc. I learned in therapy to just leave the symptoms alone and work on finding the core issues; you can chase symptoms forever -- getting rid of one addiction, you'll probably pick up another unless you work on why you are getting addicted in the first place, what it is doing for you. I think symptoms are like that, as are dreams; they are just shorthand messages and my difficult ones go away if I acknowledge them and then work on finding the actual problem.