very nice. but.. um... legal?
I liked it enough to get through all fifteen levels. The hard levels were hard enough to be challenging but no so hard as to be tedious.
The students' reaction to stairs is a bit buggy I think. Sometimes they walk "through" them. If you build stairs upon stairs sometimes you get a "double-stair" which the students can't climb over (even though it's only knee-high.)
I hope you consider making new tools -- for example an un-blocker, that allows blockers to keep walking.
If I were the original author of Lemmings, I'd consider suing you. You didn't do this for fun, you did for a corporate sponsor, as an advertising gimmick. You did it for money. Also, lots have people have said this and I'll say it too -- this isn't inspired by lemmings, it's a direct copy of lemmings. To me this looks like copyright violation.