It sounds like not coping with ASD to me - ds and dn both age 8 have ASD as do most of the family probably, so we're very used to what they find hard, but particularly over the summer it's shown how their flexibility of thinking is not there - so if dn is busy petting the cat or watching tv, he really cannot figure out how to stop doing that earlier than he had intended, which means warnings beforehand are critical.
PDA and ODD symptoms are usually manfestations of not coping with ASD rather than separate diagnoses. I found the Senco as well as GP, OT and dietician all pushing for assessment made it happen - the first assessment ds was lovely so the application was rejected, but then he managed a meltdown on the OT so I was taken more seriously after that!
Lists of what to do each day help, and calendars showing what is planned each week (ds thinks pictures are babyish, but needs the list), but the idea of marbles in jars would be too tough for his anxiety.
The Explosive Child is a great book partly because it starts from the viewpoint that "children behave well if they can" - and then how people can help them.
Has the Senco talked to the class teacher about how to help your dd learn what to do eg when given newsletters - could be she thinks it should go in her bag, but you aren't allowed to get up to put things in your bag during the day, so she has no idea what to do with it, so this is terrifying and the best thing is to make the paper go away by eating it or throwing it away. (been there...)