This special is a little longer than the episodes from the first season for a simple reason: it's a perfectly normal episode followed by a minute or so of fanservice.
Though being surely an unusual move for Yama no Susume to have fanservice, since it's completely absent for the whole first season, I didn't feel like it was awkward or forced, especially considering its mildness: in fact, given that there were no panty shots, no boob-grabbing, etc... but a very simple girl's locker room that followed quite naturally the events of the first three minutes, I think I wouldn't have even noticed it if it weren't for the nature of the rest of the show which was absolutely free from these nods to the fanbase up to this point. Anyway the rest of the episode is really enjoyable, as much as the other ones, and unless you are very prudish or very easily embarrassed in anime you will certainly be able to witness the end of this special without any problem.