Be warry of on-line proofing solutions. In my experience, once the client sees the photograph on-line, there's no longer any interest in the photo. Are they pirating the images off-line? I was using Smug-Mug pro, which disables right-click/save viewing. Clients were either really tech-savvy to get around Smug-Mug's protection, Smug-Mug was selling prints and keeping the profit by not reporting to me, or, as I suspect, the client was happy with being able to see the photo online (and share the link with family and friends).
It's a nice theory, but I'd like to learn how to make it really work before sinking more money into making it work ($150/year for SmugMug Pro at the time). I went back to paper proofs - actually went to bound paper proofs instead of loose proofs - and my sales went up dramatically. However, this was with relatively low volume portrait work, and wouldn't translate well into class photos.
Sorry I don't have a better solution, but I thought I'd chime in about the problems I've experienced with the solution you're looking into. Also, there may be cultural issues that I can't account for - Americans may be less motivated to order a photo they've already seen (and shared) online than your clients.