Be aware, my wife is a Dr of Psychology and of Criminology with 20+ years experience. Plus I have my own wide knowledge base.
Firstly it is important to realise that it is all entirely personal to you
... As in your age, height, weight, diet, metabolism etc etc, they all play a part.
Second, they use hair for a very good reason. Drugs are detectable for the life span of the hair .... Each drug has an initial different half life.
So taking the personal impact of your body, then which drugs you have ingested ..... apparently marijuana BUT what type? .... who knows what will show up in a test.
However once it is in your hair growth, then it never leaves. They exhume bodies years later in poisoning cases and find the traces of poison in the hair. Indeed, it can be like a road map,; which drugs, how much and when ..... Chemicals/drugs which appear in the hair are an identifiable marker which does not deteriorate until the hair does.
So, it really depends how close they look because it will be detectable for as long as you have that hair in essence. Even if you shave your head it will still be in the follicles if you are within the span of the drug half life in your system ..... So it will re-appear in the new growth.
The advice is grow up and do not use drugs
of any sort. At least you're not pregnant. No matter what the situation or peer pressure, you're not 12 years old are you ... so act like an adult.
Whilst I have no idea what tests the 'school' or employers use I suspect they may not be detailed enough to detect your 'once only' use .... However I suggest you keep your fingers crossed and learn to never do it again ... Also, lay off the alcohol entirely as well.