Weight loss solutions.......plz help...??...plz..?

i am a 16 year old girl...and i have taken up science stream...so i have to study all day long in order to score good marks...bt i have a very bad habit of eating along with studying...i cant study unless and until i am eating something...so i end up eating all day long while studying....and i have gained upto 4-5 Kg's in past 1 month...if i don't eat and start studying i cant concentrate on my studies and my mind gets diverted and i end up scoring less marks and if i eat and study all day long il become too fat...so plz suggest me some ways by which i can control on my weight while studying while m at home...and plz do not suggest gymming as i have to study all day long and have no such time....plz help!!....