Drab, dreary, menacing, intimidating, bleak, stuffy, spacious, rigid, rectangular, imposing, looming, fearful, dreading/dreadful, silent, tension/filled with tension etc, shabby, formidable, nervous, overwhelming, pressure, distressing/in distress, immaculate, panicking, restrictive, dreamless, atrocious, appalling, alarming, awful, daunting, provoking, blank, harrowing, gnawing (as in, gnawing on the mind), crummy, faulty, sinister, threatening, cadaverous, mournful, melancholy, uncertain, treacherous, obnoxious, grim, purgatory (my favourite out of this list), petrifying, torment, pandemonium, castigate, forfeit, competition/competitive, discipline, diligent, condemnation.
You can probably guess what kind of a scary bleak mental image I've conjured up when I hear the expression "exam hall" from my choice of words. Hope they were somewhat useful.