Chol Hamoed guest: My recollection re minhag Sanz for bochurim to wear tefillin on chol hamoed is that the explanation is that it is supposed to be worn on chol hamoed, al pin niglah and regular halochoh, as it says in Shulchan Aruch, as the Rama says (note to Charlie Hall – Ashkenazim follow the Rama in Shulchan Aruch).
Chassidim don’t wear it because of the Zohar/Kabboloh.
Since bochurim are not involved with kabboloh (kabboloh being restricted to married men, people of a certain age…), they follow the straight halocho of the Shulchan Aruch/Rama, al pi nigloh, and wear them.