Yeah this. Off topic slightly, but I had something similar some 18 years ago. Went to the school to pick my DC up, and waited outside the gate, and this one particular woman gave me a death stare every time I saw her. Actual evil hard stare. I felt uncomfortable, and after the 5th or 6th time, I started to walk up to her, so I could ask her 'wtf is wrong' and she turned her back and walked off before I got to her.
Another woman who was standing by her looked over at me and said 'what do you want Linda for?' I said 'it sounds ridiculous and like year 8 politics, but every time I see her she is glaring at me like she hates me and I wondered why.'
Long story short, it emerged that 'Linda' thought I was having an affair with her husband! I had spoken to him when he had collected their kids a few times, and had a bit of a giggle outside the school gate now and again, and a few of the mums gossiped, and the next moment, me and this man were shagging (apparently!) 😬
I said 'oh fucking hell, tell her she is being ridiculous and her so called friends are just nasty little gossips! I barely know the bloke..!'
After that, Linda didn't death stare me again, but she never spoke to me, and walked away whenever she saw me. (Embarrassed probably!) I decided to move away from where she stood. I also wanted to avoid her husband when I saw him, and so I decided to wait further down the road to greet my DC. I was glad when my DC left the school tbh!
Obviously a bit different to the OP's scenario, but proves that they may possibly be a reason for him death-staring at her.