I'm sorry not to answer your question directly, but I'm actually quite alarmed by the the whole idea...
So your friend has decided to put high-school teenagers through the torture of having to answer 160 questions? Does he or she has any idea how cognitively demanding such a task may be?
(PS, to get a faint feel for it - please go ahead to the all questions page on this site and just read the titles of the first 160. To add to the fun allow yourself 5 seconds to think how would you answer each. Then leave a comment to tell us how it went).
Has your friend done any pilots (as you normally would when venturing into such a serious questionnaire)?
I mean... we used to give students 2 hours to answer 100 multiple choice questions in exams.
How long do you think it will take?
I find it hard to believe that any academic ethical panel would approve such a long questionnaire - not only we are dealing with high-school teenagers here, but it is your responsibility to maintain the well-being of participants, and that includes cognitive well-being.
Who approved this?
Don't do it!
Need I say more?
Now that we all agree that the original initiative will never take place, here is some general advice in case your friend decides 30 questions are more than enough.
Is there a best method to show the content (pagination, infinity scrolling etc..)
If you seek to reduce cognitive load, you want things chunked and appear as manageable units of work. Infinite scrolling won't cut it. Used pagination.
What about achievements every let's say 25% of the progress.
External motivation triggers can always help, but only a bit.
If he would go with pagination is there a number of questions asked per page to give it more doable feel?
I'm not going to get into the cognitive theories behind it, but one question per page is well worth considering here.
If questions are really short and quick to answer (which they kind of must be in your case - no open questions I hope), you may wish to consider 5 of them (less physical load that way, but more noise/cognitive load).