Here's my $.02 worth.

Shoes - $15 - $20 - Walmart or Payless should have a decent shoe (preferably a simple hiking shoe maybe mid-top) that will work for school and casual purposes.

Socks - $8 - Hamilton Dry Goods has great socks. or opt for a soccer sock from Dicks Sporting Goods for $3. Sometimes they have free delivery.

For cost purposes you can wait on the sporran and sporran strap since I expect you carry a back pack to school anyway. You can also forgo the flashes as they would be over doing it for school. Add these items later or when you need them for a less casual event.

Belt - $12 - Pick up a belt from Walmart, while buying the boots, that has a nice looking rustic but simple looking buckle. 1-3/4" minimum width.

Mix-n-Match a t-shirt or Polo shirt with the outfit and you're good to go for a grand total of about $40.00.

Spend only what you need to right now to define the outfit. Do the research and wait for the deals on the rest of it.

After this "start-up" phase, get the Buzz Kidder sporran. Your calves don't appear to be over-sized so you should be safe with some standard kilt socks to add a bit of detailing (if not picky about wool these are cheap and easy to find). Your sporran will most likely come with a chain strap but consider tossing that and finding a leather strap.

Once you have the casual outfit assembled you can then start looking in the thrift stores for a jacket that can be converted into a kilt jacket for a fraction of what a custom jacket would cost. Now you are ready for dressier events.

By the time Prom rolls around you can start looking at renting a PC kilt outfit or investing in a Dress Sporran and just renting the jacket and vest.