Well at least your friends made a joke out of it instead of not saying anything (that's how you know who your real friends are).

2 words. dandruff shampoo

That is so gross!! Isn't your head itchy all the time.

I tend to get military hair cuts and wash with Selson Blue daily and avoid hair gels especially in the sun. Also I hear a rumor could be the tap water and the heavy chemicals that dry ur scalp.lather some vasaline into ur scalp the point is to get that head moist

On the bright side, now you know and you can do something about it. Get yourself some Head & Shoulders and eventually your friends will get tired of teasing you.

Take a shower

Well if you could only see it under a black light it might not be dandruff. Laundry detergent has particles in it that make the clothes seem brighter when absorbed and reflected by certain coloured lights. A black light, or Ultra Violet light, is the brightest we can see and makes these particles floresce, or glow. If under the black light was the only time you noticed the dandruff, then this may be the case.

Doesn't matter, had indoor minigolf.

There's shampoo for that, don't worry!