As long as you carry a pad or two around with you you'll be fine. You don't need to always wear one as that's unsanitary and it gets uncomfortable. You'll feel your period coming out when it comes again, so you can go to the toilet and add a pad (no need to change underwear until you're back home, it won't stain your pants) I do this and I get extremely heavy periods - it never stained in public. (I'm 17, started getting mine when I was 11) Hope this helps :)
You could even get the slenderer kind of stick-in sanitary pads and wear one in your underwear every day from about day 28, in case your new flow would begin unexpectedly and be embarrassing. But you might also get some kind of warning in the form of a few days of cramps in your lower abdomen first, which helps you get ready.
hullo....when you first start having your period they can be irregular which can be quite scary!! On average a cycle is from 25-31 days typically. So count the first day you bleed as Day 1, and then when Day 24 comes maybe start t prepare that it may come soon. At the same time it may not and that's okay...when I first started it was relaly irregular and I think my second period came on Day 60..so kind of like it skipped a month!! I know it's a scary time especially to deal with at school but find a friend to confide in if you fell confident enough to. And keep some sanitary towels or tampons in your bag just in case! Best of luck!!