Mass brief is one given to a group of candidates and usually comprises of a long brief which could be a precursor to a Pre Flight Brief or a Technical subject
Long Brief is typically a 40 minute (you'll know why) brief covering the detailed aspect of a flight exercise or a ground subject
Short Brief or Pre Flight Brief (15-20 minutes) is a recap of the essential points from the long brief ending with who will do what.
As nobody in the PPL World does a long briefs the norm is a Short Long Brief.
Apart from Flying Scholarships I can't imagine anywhere in the PPL World where Mass Briefs would be given.