I think the blairs have kindly offered to host the end of term party, maybe school hall isn't big enough. Saves the school money having to hire a hall.
The blairs are providing free food. Why should they have to pay for the coach hire as well?
Though I have read that the blairs have said if no one can afford the £10 then they can come on the bus without paying. So it sounds more voluntary. But why should the blairs be expected to pay for it all? And I'm no fan of theirs at all.