She also is only 5 and in just her first full month of school. If she was always active before school, one would expect her to be the same way at school. Experts say that it takes 3 or more weeks of constant behavior modification before a real change happens. Give your daughter and her teacher some time to work this out. Also homework is a matter of routine - which your daughter hasn't yet experienced. If she is intelligent the homework is going to be boring to her. Try to work it into a daily routine. It probably won't take her more than 10 min. to do at this point.
I don't advocate fad diets or drugs but the adkins diet will make your child lethargic within 2 weeks. Meat and dairy products will take out the energy. Take away all carbohydrates and sugars until you see she has no energy for a week. The only carbohydrate she should eat is fiber. Nature's Own has a double fiber wheat bread and it doesn't contain high fructose corn syrup. Its the only brand on the shelf that doesn't. Make her eat a peanut butter sandwitch with 2 pieces and some jelly called pollaner's all fruit with fiber. That will give her 12g of fiber in one sandwitch. Eggs (egg whites best)and sausage for breakfast, and meat and milk at dinner and you can season the meat with oregano or you can put oragano speggetti sauce on it. Oregano is a strong seditive so use it lightly. Chamomile tea, a warm bath (with lavender scents if you have it) before bed and you can put a schet of lavender under her pillow, too. The scent calms. Make sure she's getting good rest and not having bad dreams.
After a week of no energy start adding carbohydrates back in slowly until she is just right. Cold and flu medicines don't make people sleep well (make you stay in REM too much) and stay in the body for many days and increase hyper activity. Use stinging nettles for alergies, inhale boiling thyme steam for stuffy noses, hey fever, and gargle with it for sore throat. Its the main ingrediant in Listerine (thymol) without the floride so it can be drank as well.