Apart from riding schools, can you think of ANY situation where someone would need a helper/sharer/slave for their horse at weekends and school hols only? Crazy I know!
I started working with show jumpers at 14 and I was being left in sole charge at 15. I worked weekends and holidays - as long as you can prove you are sensible and reliable I think you would find someone who would love the help.
Also, don't know where about you are but if you happen to live near a university there are lots of students who bring their horses with them but go home for very long holidays and need someone to look after said animalduring that time. My last pony was owned by a student- we shared during term time and I had her to myself over the holidays-alomost three months over the summer!
Don't be afraid to ask, it's embarassing but it's really good experience. Also you will fond that once you help with/ excercise one on a yard you will be inundated with offers.
Be friendly, polite, receptive and reliable and I promise you will find something even if it takes a while.
I have only just persuaded my parents to allow me to get something on full loan after many years of riding school, working and excercising for others. Good luck!
P.S- if you have helped/ worked at riding school you will be efficient and hard working. Can't see anything wrong in that. Some of the horses may be a bit 'more' than what yoiu are used to but you soon step up a gear- I know I did. Well done for being brave.
Could you look after/ excercise horses when people go on their holidays. I'm sure their are people with children and ponies who go away for some of the school hols, and the skiing season is coming up.