First, Ukrainian «клас» is written with just one «с» because it's pronounced with a short consonant.
Second, «класа» is a genitive case form. Ukrainian nouns have several case forms, and you use different case forms to show different meanings. Genitive case form adds the meaning «of». Also, if you put «клас» in Genitive form, you also have to put «який» in Genitive.
So, «Якого класа?» would mean 'Of what/which grade?', while «Який клас?» means 'Which grade?'.
Actually, there is «класа». It's just not considered standard in Modern Ukrainian. But it was widely used in the past and is still used among Ukrainians abroad whose ancestors had left Ukraine before the language reforms. http://r2u.org.ua/s?w=%D0%BA%D0%BB%D0%B0%D1%81%D0%B0&scope=all&dicts=all&highlight=on http://www.ridnashkolacleveland.org/2014-2015%20Forms/Pytannia-pravopysu.pdf
Indeed, thanks for noting!
The rules for -а/-у are different in Ukrainian and Belarusian (the latter would have kłasa), so I sometimes make mistakes in these. +_+
I didn't know that 'класа' was a masculine tense!(What we call a 'class' in school in the West and in Western Ukraine.) The Ukrainian word 'кляса' was always of the feminine gender.
The word 'кляс' is used to denote the 'class' of a person not a 'schoolroom'.
We NEVER use the phrase "Який кляс?" It doesn't make sense and sounds 'incomplete'. We'd say instead "В якій клясі (ви/ти) є?" for "What grade (are you in)?"