Get the experience! 90% of courses won't consider you if you can't provide evidence of work with children. Do you have free/study periods in school? It may be worth speaking to your school and seeing if they will let you out on an afternoon or morning when you have these periods to help out at a primary school for an hour each week- but it's up to you to organise it. Worst comes to worst, you're going to have to try and squeeze in experience during study leave or immediately after exam time. Also try volunteering at scouts/brownies/youth organisations. If you can't get enough actual in class experience, some universities will take into consideration other youth work outside of a classroom environment, you just always need to check what in specific your uni of choice is looking for.