Not a bad selection IMO -
Trigun is a series I heard and saw recommended all the time when I first got into anime, though these days less people seem to have watched the 90s classics.
Ga-Rei -Zero- isn't as well known as the others, but personally I think it's awesome, and the first episode is especially memorable. Lastly,
K-ON! should provide some variation from the plot and action of the other two.
That's the kind of screening I liked to see in my uni's society, a good mix of old and new anime of different genres, and not just the really popular stuff that most fans will have seen already.
Aberystwyth Anime Soc met from 6:30 PM until 9 PM twice a week, though a lot of us showed up at 6 PM to socialise before the screening itself, then went to the pub after until midnight, so it was usually 12 hours out of my week. Didn't affect me at all, I just planned my work around it, even at Masters level. So unless your course is ridiculously intensive, I doubt 4 hours a week will make any difference at all.