I went from being a grade 7 student to being a grade 9 student in about 3 months also, however I had time to focus on maths only and not other subjects. This meant everyday when I came home from school I revised maths until I went to bed. Did for for 3 months straight and now I am one of the best in my school. In your situation I would say a grade 7 is definitely possible with chances of a grade 8. It all depend on if you are willing to put more time and effort into your maths over your other subjects. Hope this helps