Your dad passed away when you were a kid, your mom has been grieving and miserable because she loved him very much. A couple years later, your mom's mental health healed it was an long process but you make sure your mom takes care of herself and also you've been really overprotective of your mom from men, and you've thought your mom won't move on to an new man well..that's what you've done..

Izuku: "Woahhh..i never thought Aizawa would date someone.." He said in full thought.

Bakugo: "Hah? I never expected that old man will find love." He said in his usual grumpy tone but also surprised.

Mina: "Yeah! It's really unexpected and new of him!" She squealed excitement.

Ochako: "Even he's so strict and really straightforward towards us, it's really new for him."

Shoto: "I never knew he can be interested on romantic relationships.." He said in an calm quiet tone.

Kirishima: "It's really manly of him doing something new in his life!" He exclaimed smiling happily.

Denki: "I wonder who's the woman he dated?" He asked really curious.

Momo: "I really hoped it's an nice woman." She said politely.

Tenya: "We should be respectful to his new partner and make her feel welcome!" He said in an loud-polite tone, hand chopping.

Nejire: "Hey! Hey! I really wanna meet her so badly! I'm pretty sure she's an nice woman!" She said in her free-spirit curiosity.

Tamaki: "I don't..know, Aizawa Sensei..doesn't have social medias..he keeps his personal life in private away from the public.." He said in an quiet and nervous tone.

Mirio: "Aw man! You're right, I was hoping to see his partner looks like!" He said in an optimistic energetic tone.

Tsuyu: "He never tells us either Ribbit" She tilted her head a little to the side.

All Might, Midnight, President Mic, Vlad King, Principal Nezu, etc. The UA staff and some other pro heroes knew about Aizawa having an love partner not knowing it's YOUR mom he's dating because he never told them about her or who she is.