As a 30yr old who grew up in Tunbridge Wells, now living in outskirts of Eastbourne...
I grew up in the St. John's area of T.Wells and went to TWGGS. I absolutely hated it and was really miserable there. It was very snobby and stuck up and found that the school did not have a lot a care for students unless they were top A graders. My brother and friends went to Bennett which they were really happy with. The area itself is OK, but as a teenager there wasn't a great deal of things to do and it was actually quite boring! I've still got friends and family in the area and apparently there is quite a lot of drugs done in the parks around the area. Also - yes you can get train to London, bus to Brighton...but there's not much more around the area tbh. Also - very much a commutersville now. Also, some rough estates there too - as rough as all the other places mentioned! (High brooms, Southborough near to St.Johns and their not the best places)
People who are saying eastbourne is gods waiting room, full of old people etc...years ago maybe. But now it's quite a vibrant place, lots of younger couples and young families. Some rougher areas as to be expected with everywhere, but some lovely areas too (willingdon, old town, ratton) really good public transport. Perk of eastbourne area is the beach, the south downs, links to Brighton...Bexhill is OK too, have quite a few colleagues in their early 20s who live there and love it.
IMO I would have preferred to have spent my teenage years in Eastbourne!
Parents now live in Heathfield which is lovely, has bus links but probably a bit quiet for teenagers - although would suit some!
My husband is a teacher at a nearby private school, and he says a lot of the kids spend their time in Hailsham! There are some ok parts there, Hellingly is nice which is nearby but not sure what the transport links are like!
All the mentioned places so far are all quite different! Would be worth coming to visit them all and seeing what you think!