According to the distributed learning strategy which is based on a phenomenon called spacing effect, it is better to study classes in parallel.
Distributed practice (also known as spaced repetition or spaced
practice) is a learning strategy, where practice is broken up into a
number of short sessions – over a longer period of time. Humans and
animals learn items in a list more effectively when they are studied
in several sessions spread out over a long period of time, rather than
studied repeatedly in a short period of time, a phenomenon called the
The opposite, massed practice, consists of fewer, longer training sessions. It is generally a less effective method of learning.
Greene, Robert L.. Spacing effects in memory: Evidence for a two process account. Journal of Experimental Psychology: Learning, Memory, and Cognition15. 3 (May 1989): 371-377
Challis, Bradford H.. (Mar 1993). Spacing effects on cued-memory tests depend on level of processing. Journal of Experimental Psychology: Learning, Memory, and Cognition 19. 2, 389-396.