Hi gayatri,
Basically to tone your body up,you need to exercise. specially exercises related to the muscle u wanna tone will be helpful. now the problem that you mentioned is that you fall short of time due to college schedules. so there isnt much anyone can do abt it. but only you can help yourself. exercise in itself is very refreshing and it boost your stamina and energy through out the day, only trhe intial 3-4 days are a little tough to cope up. so if dunt have time in the evening, do it early in the morning, or afternoon depending on what time frame is available on which day. say for ex, if sat and sun is a holiday do it whenever you wish but do it regularly, on week days, if you just dont seem to find enough time at particular hour of the day thn divide your exercise session in 2-3 parts of 15 min each. as far as you have mentioned, toning is a problem but not weight, so a 15 min session of exercise focusing on one particluar part like arms will still be beneficial. although the result will not be very brisk but atleast something is better than doing nothing towards the problem.
for the gas problem, you need to watch your food. try to make a note of what food items you consume through out the day. some gas forming foods include cauliflower, cabbage, onions, rajma, chole, chana dal, fried foods, too much of potato, spicy food... so avoid them and also note the meal timings. if the gap between 2 meals is a lot, then gas would be formed. so try to incorporate small meals if nt much you can keep a pack of oats cookies , or roasted channa in your bag and have it whenever possible. also check what you have for lunch and breakfast, try to incorporate a fruit in any one of these meals.