The secret is don't worry!You sound a bit like me - im 15 too and play flute piccolo piano soprano sax and and electric guitar. I set aside about an hour a day for music practice, sometimes far less, sometimes far more. Im sure it depends on how ambitious you are as to how dedicated to practise you need to be. Im currently working for grade 7 flute and 3 or 4 piano - i don't do grades in the others, i play piccolo/flute in my county youth orchestra, sax in my school sax band and guitar in a band with my friends. To be honest with you, i end up practising in the rehearsals of these groups anyway, which means i dont have to worry so much about practising at home. Its certainly more enjoyable when you rehearse with others!I wouldn't worry too much about timetabling, if you play for fun when you want then you'll progress far more than if you force yourself to do it at a set time. I sometimes feel like this, you're not alone. I find that because of my grades on flute and piano, the other instruments are a lot easier to play and i dont have to worry about practising them as much (in fact, the last time i practised guitar whilst not at band practise was about a month ago, and ive still got better!)So dont panic, prioritise - if your working towards your grade 8 and 4, do most practise on those instruments, but listen to your teacher - if they really think it will be too much they could be right - you sound really talented anyway - you can always split the exams up do one in the spring term and one in the summer after all, thats only a gap of a couple of months, so you'll have them both done by september. Im not saying that you shouldn't do them at the same time, but it might pay off by spliting them slightly. Good luck with whatever you choose to do.Anna x