Учащийся - reflexive adjective?

I came across this word today, and I've never seen another Russian word that behaves this way grammatically. It is an adjective or adjectival noun, plus the ending of a reflexive verb.

The part before the ся is declined like a normal adjective, but then the reflexive ending is attached:

учащийся - masculine nominitive singular - "the male student / schoolboy"
учащаяся - feminine nominative singular - "the female student / schoolgirl"
учащегося - masculine genitive singular - "of the student"
учащихся - genitive plural - "of the students"

So would you call this a Reflexive Adjectival Noun? :-)

Are there other adjectives that have the reflexive ся ending? Or is this a one-of-a-kind word?

Yes, there's a whole group of words like the one you mentioned. -> Adjectival participle - Wikipedia, the free encyclopedia

Here's more in Russian: Причастие (лингвистика) — Википедия

In a nutshell, it's roughly equivalent to the "-ing" suffix in English, e.g.: "studying".

Virtually you can derive two adjectival participles (present and past) from any imperfective verb and one (past) from any perfective verb. Verbs can be either direct or reflexive. Plus sometimes passive participles are possible.

Example:
учить - to teach
учащий - one, who teaches (present active a.p.)
учимый - one, who is being taught (present passive a.p.)
учивший - one, who taught (past active a.p.)
учёный - one, who was taught (past passive a.p.)

a.p.-s can be used as adjectives and as nouns (like if you add "one" in English). Rather often they become regular nouns but always decline as adjectives. For example "учёный" also means "scientist".

Interesting. A participle formed from a reflexive verb keeps the СЯ at the very end. I would have expected it to be "учащсяного", not "учащегося". :-)

Спасибо за помочь!

учащийся = учащий себя = (smb.) teaching oneself
учащегося = учащего себя = of (smb.) teaching oneself