Charter for 21 from Tsawwassen to Science World?

I'm escorting some kids from Vancouver Island over to Science World this late spring/early summer, and I'm looking for a reasonably priced charter that can collect us at Tsawwassen and drop us off at Science World, then reverse the trip the next day.

Most charters I've spoken to have a minimum booking requirement of 6 hours so it keeps bumping the cost too high for us. But I do not look forward to bringing overnight gear for 21 kids onto public transit while also trying not to lose anyone.

Anyone with charter bus suggestions, or cautions on who to avoid?