Technically, Sensei is very smart and quite observant. He answers hard questions casually at multiple events, including that time Touka tested him and the recent trivia challenge at the bar where he concluded that he answered the remaining questions.
I'd say he's in par with Makoto and Nodoka if it weren't for his laziness and circumstances considering that he got a really good academic record in the past.
Honestly, big brain and big dick. He might think with his brain or with his dick, that's it.
Just to add on to the intelligence ranking, Noriko is officially second to Nodoka based off tests at their school, was tutored by Sensei, and still seems to admire his intelligence, much like Makoto.
Overall: it's reasonable to consider Sensei #2 when it comes to overall intelligence, when he actually tries, in game. Only Nodoka has actually outdid Sensei when it comes to notable tests of knowledge as far as I recall.
Then again, I'm not sure where Wakana actually ranks, albeit Noriko seemed to expect more from her and Wakana considered Noriko "suspiciously smart", so I'm not sure if she's actually more intelligent than Noriko.