I'll chime in I suppose.
There are two ways to go about this. You can take the approach that looks more like a 400/800m runner approach. It is often a lot less volume, a lot more intensity, and seems to work really well for runners with a lot of natural speed. I can't tell you much about this approach, but it has been talked about a little on this thread already. It seems generally more complicated, though.
I prefer to go about the 800/1500 approach, especially if you have decent natural speed but not enough to be equally competitive at the 400m in the long term. And even then, using a more distance oriented approach has helped me improve my 400m time as well. I am assuming you are a north american student and that the summer won't include much if any racing.
Fall XC, Winter Indoor Track, Spring Outdoor track. If this is incorrect lets us know.
1st off, I would run XC for sure. It is paramount, and even a 400/800 runner like Johnny Gray says XC is really important. Use this summer to prepare yourself for the distance running that XC requires. Up your mileage, first and foremost. I can't tell you how much to run, I can only say to run more than you were before. Consider adding a short run a few days a week to get some doubles in.
2), keep in touch with your speed. 2-3 times a month, do hard hill sprints. Find a hill, decently steep (but not a mountain), and run 6-8 hill repeats that take anywhere from 5-10 seconds to run. It doesn't matter too much, provided they're not much longer than 10 seconds. In addition to this, do 4-6 strides a couple times a week after easy mileage. For the sprints and the strides, just make sure you're taking enough rest to feel good. If you feel like you're tightening up on any of these then you are going too hard or not taking enough rest. We don't want to build much, if any, lactate acid during this base building phase. We just want endurance and top speed work.
3) Aerobic quality. Tempos. Steady States. Fartleks. These are the bread and butter workouts during an 800m runner's base phase. Even a guy like David Rudisha does these types of workouts often in his pre-track phase. For the tempos, I wouldn't hammer them too much. Running near marathon pace (plug your current 5k fitness into an online pace calculator to get marathon pace) is great work to be doing in the base phase. I often won't pick out a set distance to run these tempos over the summer, I will just run them comfortably hard and when I feel like being done, I'll stop them and cool down. Sometimes that's after 3 miles, sometimes it's after 7 miles. Listen to your body. Progression runs are also a great way to get a good volume in at a good aerobic pace without being too uncomfortable or difficult. Fartleks are good for getting in some higher quality running without digging yourself into a hole. Have fun with the fartleks. Vary the pace, length, rest, etc.
4) Some easy speed work. Head to the track or a grass loop you can find and do some work at mile and 800 pace. Just make sure it is short in duration and includes a lot of rest. An example might be something like 8x200m @ goal mile pace with 2 minutes rest. Should be a super easy workout, but will also keep you in touch with your middle distance speed.
5) I would recommend lifting, also. I have personally had success lifting relatively heavier weights for 4-8 reps, doing the main lifts such as squats, deadlifts, bench, incline bench, pull ups, and a lot of core. At the very least get a good core routine, but I feel like lifting can help if you have someone to help you with form.
Okay that's enough for now. This is just what I believe works well for middle distance guys building base. It's pretty basic. Run a lot. Do some sprints. Do some faster distance runs. Run slowly in between. Don't overthink it.
The cool thing about running is that there is a million different ways to get to where you want to go. Just put in the work, be consistent, and you'll be well on your way.