A somewhat strange thread as hardly anyone has mentioned, you know, sprinting. At least the other stuff is taken care of like drills, weights and flexibility.
You have to realise that 100 / 200 training has traditionally a different focus than 400m training. Anyway you can worry about that later. Elements to start you off with:
1) 100m accelerations: Gradual accelerations ending at about 400m pace. Never all out early in the season. If you do 10 of these with a slow walk back recovery twice a week it is a good start.
For each 100m rep think about another element of form while you walk back, e.g. knee drive, ankle extension, relaxed upper body, lower drive position etc. It's amazing the improvements you can make concentrating on form under fast relaxed speeds.
As the year progresses these can get quicker - or you can make them 150's or 200's etc.
2) Fly-ins: Once warmed up you can do these. Jog into the start and then explode into the sprint - you will reach top speed within about 30 - 40 metres - as soon as you do decelerate gradually. Do 5 - 6 of these with plenty of rest.
3) Weights - at college between drinking bouts we did some strength work, but mainly focused on dynamic / elastic strength - for example on the spot high knee runs with barbell, step-ups with barbell, jumping quarter squats with barbell, two footed jumping over hurdles.
4) Technique drills - these will help develop leg speed, technique and will assist with the natural development of a sprinters range of movement.
Good luck mate. Do all this properly and consistently and those times will take care of themselves. 6 months of this and you'll be ready for lactic acid clearance workouts (okay maybe not at 15) and jaccuzis with the cheerleaders (ok, maybe not....etc).
Never, ever underestimate the power of consistency, determination along with a coherent plan.