I did City Walking Tour with Matin McCrossen’s company. They were fast paced, included humour & friendly - I think would prob work for young people.

When we walked the walls we did it independently so cannot advise on which is the cheapest tour or the best value. I would suggest you contact the Derry/ Londonderry tourist board by googling it and although they will not tell you which is the best they should be able to give you the details of each one. It may also be worth looking on Tripadvisor for attractions in the city. Beware though as some of the guided tours will be IMO ridiculously expensive for what you get. There are some information panels on the walls but 50 people would not all see the same one at same time if you want to keep the group altogether. Mind you I doubt one guide would be heard properly addressing such a number either. One plus is the walls are very wide and the group could walk 6 abreasteasily and still not block access for other groups.