WOW! 50 bikes! I am currently thinking of trying to get something together to go to my head of dept and hopefully business manager to see about the possibility of offering MTB as part of PE. I would say you could start with an introductory lesson, basic bike set-up for individual pupils (saddle height etc), tyre pressures (for different terrain), bike parts and their functions. A lesson on bike maintenance would also be needed as out on the trails you will undoubtedly encounter snapped chains, punctures, buckled wheels, brake pad/disc wear etc. MTB basics would include drop-offs, wheelies/manuals, climbing, downhills, techniques for roots, mud, rocks, switchbacks etc. Also, cornering, small jumps, stopping, bunny hops- all required in the great outdoors. Are you planning on riding at trail centres or in the countryside? A lesson on public rights of way is a must, details can be found on the forestry websites.