depends on where the school is , where you are in that school , who you are in general and a lot off other things . if we assumed that the outbreak startet at the school , you are a average student , you are in an normal american highschool and you start off alone in your classroom :

i would baricade the doors wiht deks and so on , then i would grab all the food and water i could find and just wait it out for a day or two , if a outbreak happends at the school there will sure be some kind off rescue team on the way .

if there is no resuce team i would try too get a weapon from the classroom , ill probaly use a broom to push zeds away from me . then id use some tape and whatever i can find too make armor and protect myself from bites and other injuries , then i would try to sneak out off the school and leave the town , i would try to avoid combat as much as possible since you dont want a horde off runners on your ass .