Have done it twice as a primary school teacher its not easy in the beginning but gets easier.
Loo issues:
You need to keep yourself hydrated so drink loads earlier enough to have a long wee before class starts.
As running to the loo is an issue with no TA drink water about half an hour before your break so you go properly.
Speak to senior management if sympathetic or teacher next door re loo runs.
Sickness:
Have a box of plain crackers, ginger biscuits etc in class well stocked just so you can nibble on and any mints that can temp aliviate nausea.
Try and sit down as much as you can I know can be impossible, I found that with my younger class I made sure I had a cushion on my chair and for back for story time just before lunch and just before end of the day so I knew I could sit down.
With my crazy Year 5 while I was teaching and or whole class if feeling tired etc would pull up a chair and rest one knee on while standing I know it sounds a bit odd but I dared not sit down too much as they were a tough class.
I also stuck to strict timetable with the class with group work so they knew that on certain days I would be working with certain different groups and at least knew I could sit down even for 10 minutes with children.
Again drink a bit before lunch as you can then go.
I would also if you can mark as you go along through the day you probably do this but it means you can spread it out over the week.
You'll soon find out something that works for you.
Sorry about the obsession with drinking water and loo but I did not and ended up with a nasty UTI in both pregnancies which defeats the object as you end up on medication and drinking even more.
Don't try and do too much ( you will anyway as we teachers tend to) try and get lots of help and try not to stay late every day..
Sounds cynical but its true if you're off sick or you or baby is unwell they'll just replace you so think of yourself and baby.
Good Luck