You can always ask your partner in lab to set the treatment up but not to thrust. Your classmates will always be mindfull of your request.
On the other hand, you cannot do the same if one of the instructors or one of the OMT fellows come over to show your partner something and he is using your neck for the demo. They will set the HVLA up and thrust, unless you have some documented medical condition which is considered a contraindication for cervical HVLA. The same hold true for practical exams. Your partner will diagnose a dysfunction, on you, and treat it.
I was not too keen on having my neck cracked too many times. If I asked my lab partner to set up the treatment but not thrust, there was always a willing volunteer next table that would let my lab partner thrust on his/her neck. Many people really enjoy having their necks popped. Other students will come up to you and ask for cervical HVLA, so you will have plenty of opportunity to practice. I personally don't like it done on myself because I have some long standing dysfunction and it doesn't help me at all, just makes my neck sore for several days afterwards. I prefer other techniques for my cervicals. As it is, I only had to have my cervicals popped 3 times by other students.
Unless you really have your neck popped too many times, or you have a medical conditions that makes your cervical spine unstable, HVLA of the cervicals is not going to hurt you. The rare cases of people stroking out from cervical manipulation, or having a carotid dissection, happened, for the most part, when the neck was hyperextended. Hyperextension of the neck for HVLA is a big NO-NO. Don't do it on others and never let anyone hyperextend your neck for HVLA. When in doubt, ask one of the instructors or just "pass up" that chance of being manipulated.If you notice that your partner is particularly inept or ham fisted, I would also ask not to be popped.
Don't let this deter you from pursuing osteopathic medicine. There are many wonderfull benefits in knowing OMT.