From our past experience on "group" lessons:
Getting the most from a learning experience as involved as pitching or hitting requires intensive one-on-one training on an ongoing basis. IMO, there is no way a student is going to get maximum benefit from a "gang" one hour lesson with each kid vying for attention. As an evaluation, that scenario may be fine, but giving quality instruction is impossible. An instructor must give 100% attention to the student for the entire lesson - otherwise the student (and her parents) get short-changed. Getting a $5 discount for a 15 minute lesson was not perceived as a bargain to me. I would rather pay the full amount and get 100% attention for the full 1/2 hour. If I was looking to cut corners with "bargain" pitching/hitting lessons, I'd go to Wal-Mart.
We almost always opted for the one-on-one lesson, but the next best set-up we experienced was with 3 students in rotation using three pitching lanes side-by-side. It worked this way: A student would spend roughly 20-30 minutes warming up in lane one. Then the instructor would take that student and work with her in lane two for 1/2 hour. Then the student would move to the third lane and work on what she had just learned for 1/2 hour. The student only paid for 1/2 hour of instruction, because that's what they were getting - BUT they got to throw for an hour and a half, including time for a full warm-up.