I've never heard anything bad about Dive Aruba or Clive (the owner/operator). Check with him. I'm sure Pelican would allow it also as they seemed pretty nice (I dove with them). Also heard good things about Unique Watersports.
For students, the Pedernalis dive is a good beginning dive for them. average depth is 10 to 20 feet and has part of a wreck torpedoed from WWII there. Lots of fish life as the snorkle boats feed the fish. Due to shallow depth, lots of sunlight and happily fead fish means great photo op site also. He can also dive them on the Antilla, a 400 foot freighter in about 55' of water but the wreck protudes the surface, so he could keep them shallow if needed at first.
Other shallow sites would be Arashi and Malmook reef areas, but I don't know of scheduled dive boat trips to there, but the snorkle boats go there. Clive might do it as he runs a smaller boat than Pelican, etc.
Redsail is the other big operator on the island. While I've never used them, most reviews of them include the words "cattle boat" so I've steered away from them. They tend to cater ot the Marriott, Hyatt, and Cruise boat crowds and add them all up on one boat and it gets crowded.
hope that helps. -mike