if i was you i’d keep it informal, once you set up anything official you’ll need qualifications, insurance, risk assessments, safety checks on all equipment even if it’s not yours, pre ride visits, accident forms, safeguarding policies and procedures, CRB checks.
if you still want to go ahead with a MTB club might be worth contacting your local council to see what assistance they can offer you and possibly act as an umbrella organisation, try the youth service or for the bigger councils they might have an outdoor activity co-ordinator or even an outdoor education team.
sorry if this sounds negative but having worked in the industry i know the hassle of organising rides for young people and always find it easier to just get out and ride.