My suggestion to you is to see if somehow you can find a doctor that is previous military. You might have to do a lot of research to find one. The reason why is a military doctor understands the military regulations. You can say at this age you want to attend either an SA or ROTC and you are concerned how this will impact you later on from a DoDMERB aspect. They will get what your concerns are and can write in your records their diagnosis in a way that may help later on from a remedial or waiver process. They would also know which prescription may become an issue later on, compared to other doctors.
I know that there have been several posters when concerned with the fact that their family doc wrote that they may have asthma on their records, they went to a specialist that was previously a military doc for the spirometry test. Although all of those posters did get at the very least a remedial, it appeared to me that because the doctor they went to on their own were able to move the process along faster since that specialist as an ex-military doc knew exactly what to write on the record that it moved faster through the process. I believe in one case the poster was told to take an exam with their doctor, and for them, instead of waiting weeks to get an appointment, they just called the doc and had the records faxed over the day they got the letter. Faster in and the better off because later on the pile gets bigger. That is just my opinion from what I have seen here.
Finally, if it was me, I would not wait, I would go now. Think of it like chicken pox. If you go 2-3 weeks after you have chicken pox, the pox would be healed and it would be harder to diagnose 100% for sure than if you went the day the pox appeared on your body. It is easy for my DD to me diagnosed because behind one of her knees was where she always gets eczema and even now when she tans, the back of that leg where the eczema occurred does not tan like the other leg. It tans, but it is much lighter, probably due to the skin.
~ Her eczema is also due to allergies. It took years to figure out what caused the flare ups, and found out only because my cousin that has psoriasis made a comment that he could not eat shellfish because it causes a flare up for him. I put 2 +2 together and realized that her flare ups happened a few days after she would eat shellfish. She still eats shellfish, but she knows that she better have prescription ointment filled because it will flare up.
PS. You are still young enough that even if you go now to the closest dermatologist for a diagnosis, and use the next few months to find a doctor that has a military background, it can be turned around because you are 13. Let's say the 1st doctor gives you a script that is not allowed, but the next knowing that it is not allowed changes it out, than you would be off of it for 4+ years before you take the DoDMERB. It can negate the 1st script.