Figures of usage aren't easy to find, but one study, for example, cites the Diccionario de Frecuencias de Justicia as listing 1563 verbs, which are distributed as follows:
- Class 1 (-ar): 1211 verbs (77.5%)
- Class 2 (-er): 178 verbs (11.4%)
- Class 3 (-ir): 169 verbs (10.8%)
(The sum of the percentages doesn't add up to 100% because of rounding.) This distribution seems to have existed for a long time. The numbers are about what I had gathered they should be from previous reading on the matter.
The -ar class is special and probably is treated as special for students because
- It's by far the class of most verbs that you'll encounter;
- It's the class with the least amount of irregular verbs;
- It's the most productive verb class, meaning most new verbs in Spanish are only coined in this class.¹
I understand the numbers above are verb counts, not usage frequencies. In actual usage, of course, you'll find many instances of verbs of Class 2 and 3 like ser, haber, ir, venir, etc. that might shift the figures a lot.
¹ As pointed out in a comment, the suffix -ecer is regularly used to coin new verbs out of adjectives (often together with the prefixes a- or en-), so the -er class is also productive.