You cannot and should not keep the same tampon in all day at school.
When wearing a tampon at school or anywhere else, you need to change the tampon out every 4 hours to 8 hours to prevent infections, like Toxic Shock Syndrome.
You should plan on changing out your tampon at least once or twice during the school day for safety and hygiene, even if your menstrual or period flow is light.
Bacteria can easily build on a tampon, when the tampon is left in too long and increase your risk of infection.
Leaving a tampon inside you longer than 8 hours also raises your risk for Toxic Shock Syndrome, which is a rare, but serious illness.
And leaving the tampon in too long or all day can also cause abrasions, which can allow tolerable bacteria into your bloodstream, where it becomes intolerable.
Prolonged the same tampon can cause dryness, abrasions and irritations.
Carry extra tampons with you in your bag, so that you can change it out when needed and you can also consider using period underwear or even menstrual cups for longer wear if you can't change your tampons frequently enough.
Although you should aim for changing your tampons every 4 hours to 6 hours for best health and hygiene, but never exceed 8 hours without changing your tampon even if your flow is light.
And change the tampon more often, such as every 3 hours to 4 hours if your period flow is heavy.
And always wash your hands with warm water and soap before and after changing your tampon.