The main character sets the football shed on fire. She wears gloves and takes care of the gas can. Could she possibly be found out? What else would she have to do to avoid being found out?
What time does she do this, is the high school campus near lots of houses? A little more information is needed... If she does it in the middle of the night and if it is anything like my secondary school, they have movement sensor light things so anyone living opposite would see lights come on. She also needs to make sure her route into the school grounds is safe for her, will she struggle getting though/over fences, could she leave hair or clothing scraps? Also is there security cameras and alarms she could set off? If there's none of this its not a very secure school but she would probably get away with it, if these things are present, she has a higher chance of being caught.
How old is your MC? I ask because obviously there is an age restriction on buying fuel. Also like @I.A By The Barn said more info is needed is it a small village or a big town?
You've mentioned that she wore gloves and took care of the gas can (presumably disposed of in a way that it can't be found). But there are many other sources of evidence that could link her to the crime. Footprints in the mud could identify her shoe size, brand, style and gender. That would narrow the search down considerably. The pattern of burning could indicate how tall she was by how far up she was able to pour the petrol. A possible motive could narrow down the search even further. The character could slip up in an interview and say something incriminating. Somebody could come forward and inform the police that a can of fuel was stolen from them, which could narrow down the search geographically. So yes, there are many ways that she could be caught. You should try to incorporate several pieces of evidence which are used to narrow down the search and ultimately identify the perpetrator, as this will make for a more interesting story than 'someone saw her do it'.
I don't think there's an age restriction on buying gas where I am... that may be a regional thing. It might help, OP, if you tell us if you want her to get away with it, or want her to get caught. I think either could plausibly happen - this certainly isn't a totally "unsolvable" type of crime, but it's also not an open-and-shut, totally obvious thing, either.
Usually the biggest piece of evidence is the psychology of the arsonist. The first person that the police will go to will be the guidance counselor. In a small high school like the one I went to (graduating class of about 180) the counselor knows every student fairly well. That'll provide maybe a dozen suspects. Kids aren't exactly master criminals and are susceptible to simple mind tricks that police use to get the truth. Arsonists, also usually return to the scene of the crime, they want to see.
It would take a very strong-minded person to say nothing at all about it to her friends or on social media. That would go several times over for a teenager. I don't mean confessing the crime, necessarily, but things like sympathizing with the perpetrator, who she pretends is someone else. You know, "I would never do that, but those jocks deserved it." Enough of that talk, and folks will start to wonder.
