My DD behaves similarly, though its usually just the one meltdown per morning and afternoon.
I make lists for her. Have you tried this? My DD preferrs written lists but you can get the visual timetable symbols off twinkle or other websites.
I have a list of "what's happening this week", just a few bullet points for each day e.g Monday - school, after school club, Tuesday - school, swimming lessons, Wednesday- school, grandma collecting from school etc.
Then each morning she has a list of what she needs to do and the order it's happening in. And I write a list for her that she keeps in her school bag about what will happen after school broken down in small steps e.g Mum will collect me at about 5pm, walk home with Mum, her home and watch TV, eat dinner, bed.
School also give her a list of her day, plus an extra list for the end of the day of the things she needs to do e.g get bag and coat, line up etc etc.
It was explained to me that when they have such a high state of anxiety even the smallest uncertainties can disregulate them and cause a meltdown, so given them warning and detail can help with this.
It doesn't sound like the school are that supportive of your DC and you if they are simply saying they don't see a problem at school. They should know how common masking is and should be making adjustments accordingly.