If it's GRP with timber trim and the timber is as sound as it appears in the pix. then it's mainly a matter of a thorough shampoo and rinse using a soft brush followed by sanding down the timber and finishing it with something suited to its quality e.g Burgess's for cheap & cheerful or varnish if you're ambitious. Mostly manhours involved [free?] plus, say, £50 for brushes and finish. Biggest cost will be the oars if they are missing.
Follow your nose. Have you floated it to identify any leaks?
Clean the bloody thing out.
Have a good root around all the woodwork.
Where the centreboard case fits on the hog (for want of a better word) will probably need attention, best to actually remove the case and do it properly. Where the stringers are fixed look like they will be through-hull fixings, so again, remove and re-bed and seal them. If you are confident they are sound and not rotten on the inner (unseen) face leave them.
Replace any wood that seems a bit feathery.
The rubbing strakes and the lower strakes may need replacing. Don't use filler, even marine filler. Only epoxy will actually stick and really it'll be easier to replace the wood.
The interior grp looks like it's had a bad paint job It takes a load of work to get the old stuff off. Don't use normal paintstrippers like Nitromors which will be harmful to the grp.
In fact probably best to keep away from chemicals.
Not wishing to sound mean, I wouldn't allow any sea-scouts to work on it until they had been taught a few simple techniques, and certainly not to work on it without supervision.
I have been astonished by how inept most youngsters are these days when it comes to simple maintenance tasks. I'm not saying they are rubbish, but that they have never been shown.
I have seen a scout work party that had run short of paintbrushes dip their hands in the (gloss) paint and spread it about with their hands when painting a container.
They can invent new ways to frighten you.
Looks as if it is designed to sail. Do you have any mast/sil/rigging? If not and you want to sail it will be expensive unless you can find a s/h rig from a hull being scrapped.
Defo designed to be sailed, with centreboard case & mast hole in the forward thwart. Cat rigged (gaff or sprit?) lugger at a guess. Yup, clean it first, jet wash used carefully will do for starters, then careful use of a penknife blade to test the timbers.
Centreboard is probably a galvanised plate & spars could be simple 4x4 pine planed octagonal. Almost any old dinghy sails could be cut down & stitched together for the sails. Probably worth stitching an old rope around the edges nder tension to help carry the loads. looks a fun winter project.
As Lakey says, you will need a sensible adult with some experience to keep an eye on the dafter lads! Any of the Rangers/ Ventures got any wood working skills?