Is this an amorphous terror, or do you have specific concerns about what she might do with her anger?
She's probably already done her worst, frothing herself up to try to pull in Flying Monkeys (texting anybody who will read, buttonholing anyone who will listen) - your best bet is to think "so what? If I can she she's batshit, so can anyone else." & let her crack on with it.
Unless you are scared of a physical altercation, don't hide behind your phone. Just act as you would normally. If she comes as you still frothing, helpful techniques can be:
- Hold up your hand as is stopping traffic. Politely say"Can I just stop you there?" ... then impolitely turn on your heel & walk away.
- Give her full eye contact, stand tall & still, fold your arms across your chest & just watch her like she's a David Attenborough exhibit. Say nothing, respond to nothing, & let her rant.
She'll either run out of steam or other people will start staring & she'll realise she's the only one making a twat of herself.
- Say "Take it up with the school, because you are being ridiculous & probably need a teacher to tell you so."
Say nothing else (Broken Record technique) - just this phrase on repeat.
It's a useful one if you feel prone to voice-wobbles when confronted.
Outside of that - why are you still accepting her aggressive messages on your personal device?
You have no more business tolerating that than you would tolerate physical abuse.
Stop trying to 'defend' yourself - you, & your young child, have nothing to defend.
Are you sure you want him playing with the child of such an aggressive parent - do they play out of school, & that's why you have each other's numbers? I'd put a stop to any of that, I would not want my child potentially subjected to her anger.
Do you think you are 'people-pleasing' ie scared of her anger once she sees you blocked her? Aggressive people don't stop until challenged. Challenge does not equal confrontation. Confrontation can make an aggressor either back down, escalate, or seek a new target. I can't tell you which she will choose, but I CAN tell you that your "path of least resistance" is counter-productive & will get you nothing but more of the same. So choose an option & just do it. The first time is difficult, but up until now, all this woman has taken from your response is that you will KEEP TAKING her aggression - so she has you down as a handy receptacle for her bullshit, who she can use as a verbal/text punchbag without fear of pushback.
So decide: do you want to live in fear of her - essentially - doing nothing more than making a prat of herself for all to see, or do you want to woman up & calmly say "that's enough now."?