I would doubt that there are any coach trips to Thorpe Park at this time of year. It closes for their winter break 3 weeks today (9th November).
If there are any trips available, the Portsmouth Tourist Information Centre should know about them. Phone them on 023 9282 6722 to ask if they know of any. If they do (which I think will be very unlikely this late in the year) ask them for the telephone number of the relevant company, so you can see whether they'll take you.
The only other alternative would be to travel by rail. It costs �22.50 for an adult off-peak day return ticket from Portsmouth to Staines. (Half price for under-16s). When you buy your rail ticket at Portsmouth & Southsea station, you can buy an additional ticket which includes both the bus from Staines to Thorpe Park and your admission fee. (You'll all need the adult ticket because the child price at Thorpe Park only applies to Under-12s). The price is �27.50
That would make the total cost for a 16-year-old �50, with a 15-year-old paying �38.75. However, although buying that combined bus and entry ticket at Portsmouth saves you money on the price of paying for the bus and admission separately, you could pay slightly less if you pre-booked your Thorpe Park tickets online and then paid for the bus separately. ('On the day' entry to Thorpe Park is �34. Online tickets cost �24. If there are at least 5 people in your group, that comes down to �22).
Chris