The best way to stay protected against any type of period emergencies is carrying a pad or a tampon with you at all times. But, if you don’t have any pads at hand and need one immediately, don’t be shy in asking a friend, a teacher or an elder student at school to help you with it. Pads and even underwear is usually available in school and asking for help is nothing to be embarrassed about.
Also, on the days when you are having minor spotting or vaginal discharge, wearing a ‘pantiliner’ a special type of thin pad - is a good idea.
A short term solution until you get a pad would be folding a tissue paper and keeping it in your underwear. This would help avoid staining on your uniform until you find a pad.