All you should do is find a workout that strenghtens your legs and do it daily + you should run at least two laps at full speed and when you think your stamina is out run a few meters more every time. And dont be anxious just belive in your self.
Yeah I remember being in a similar position. Though I went with being the pace maker so my team mate could break the inter-schools record. Strangely both team mates did and came 1, 2 and thanked me. I finished 6th although I was meant to drop out, but I just felt comfortable.
Anyway what I found helped is to clear your mind and just focus on your breathing and one foot infront of the other. It's a race sure but your biggest opponent is exhaustion, so at the start keep your adrenaline in check and just short "boost" to your comfy pace and good position on the track. So keep a good rhythm, breathing, strides and your pace will come naturally. Then last 80-100m empty the tank. Everyone respects a sprint finish no matter how long it takes to get there.
Also remember to just have fun and enjoy it. You got picked so you're the best! Think Forest Gump and Finding Nemo, "Just keep running/swimming"
So I'm just interested to hear how was the race? Hope you survived and had fun, if not? Well you now have a story to tell and the memory of the time you ran the 1500m for your school