I've just got it here at BKK. The desk is in the immigration hall at the bottom of the escalators in the corner. No queue but can imagine it's bust once passenger numbers increase
Check the Thai Government website as it will detail this. In UK for example I think bona-fide students can work up to 10 hours a week but check what the rules are here.
Well I tried today at Chiang mai immigration and they were happy with everything until the last person to check my paperwork and wouldn't issue the extension as she calculated that I couldn't apply until tomorrow as my Visa runs out on Aug 2nd and 45 days before is June 19th not June 18th despite her colleagues agreeing 45 days before is June 18th...... what a bummer.