My daughter just had a two day prep.
She has used Miralax and Gatorade in the past (Miralax is called Movicol in the UK, I think) + Dulcolax. That prep didn't clean her out enough. It was one day of prep with a light breakfast and the laxatives starting at 12 pm.
This time, we did sort of a two day prep. One week prior to the procedure, we started a low residue diet and also added 1-2 capfuls of Miralax daily. The day before the procedure - NO breakfast (that was a big change) and clear liquids all day. Only had broth, popsicles, jello and clear juice. Well, and Pedialyte and water.
We started Golytely in the evening - like 4 pm. We did 64 oz that evening.
The next morning (and by morning I mean from about 2-4 am), we did another 64 oz of Golytely. Apparently a split dose prep works better but she still wasn't cleaned out enough. Her prep was called "fair."
So I guess we need a plan C, since plan B failed!
In the past, with a 1 day clear liquid diet/Miralax prep, my daughter went to school in the morning. Not with every scope, but with most of them. Since we didn't start prep till 12 pm anyway, she preferred to go to school - that way, she only missed lunch and 2 classes.
I'm surprised they are allowing milk as that isn't usually allowed.
We try to push as many fluids as possible - that way they feel sort of "full" instead of hungry. I'd do that, especially the day she is attending school. I'd send her with several options - broth in a thermos, jello and a popsicle for lunch etc.
I'd even experiment with different milkshakes before the prep - there may be a flavor that she can tolerate. You can change the taste by adding strawberry syrup, honey, maple syrup etc. Keep it cold and use a straw - that will mask the flavor. You can also put it in a Zoku slushie maker - look it up on Amazon.