I have a son in an indie not far from MTS and the' extras ' are what you have to watch out for. This can be uniform items especially games kit
that gets lost, school trips, (DS going on ski trip in 3 weeks time @ cost of Â£1090),music tuition etc.
Also fees go up from Y9 by nearly Â£3000 per year from Â£12000 to Â£15000
at my son's school so check if the same applies at MTS.
With regards to exotic holidays, I haven't heard of any of my son's friends going anywhere special - even the really well off ones, so don't worry too much about that.
At my son's school uniform costs were fairly modest , books and school lunches are included in fees.
Another thing to watch is coach fares if your DS will be travelling that way - I think it is about Â£500 per term.