Nah, Ikumi should be into guys, but she's an adult (I agree it would be an interesting concept, but I don't think she's into girls).
You can see it by her reaction to Sou first time they meet, or when he greets her in the mornings. She isn't used to a boy treating her (an oba-san) nicely and politely. But she's used to seeing boys. Obviously, as a teacher she would have met several thus far over the years. And look at her when she entered the class after the hugfest... she's not fond of the female pheromones but she noticed Sou's scent mixed in. Yuri? Never, but compared to the teenage girls she teaches, Ikumi is a mature woman who is past her adolescence - she's more collected and has mastered the academy's "Ladies Education". How would she educate aspiring male guards otherwise? Not to mention, Ikumi's job is on the line, if she were to make a move. That also plays a part.
But she's by no means unaffected. It's more like with Kana's sister Akira Konoe. She told Sou how it usually works for women like her - since men are relatively rare, good men all the more, even rich, well behaved ladies like Akira get dumped very easily because men still find them intimidating (not visually, but because of her power/wealth). The usual woman, someone like Aikawa Ikumi, has way fewer chances to even meet guys. So for her, having Sou in her class is a big win. Not only can she enjoy having him around, but she also gets bonus payments if he stays. No way she will risk that easily. Her decision to not make this a school event also comes from that...