You have said you will be at the school at 1pm, so be there.
As calmly as possible take all your kids home.
Talk to them. Tell them that you regret calling their Dad to take them on Friday, and you are sorry.
Tell them you love them to bits, but you are a human being and rows on the school run push you over the edge.
Ask each one in turn how they think you can all work as a family to stop this happening. Rules for discussion: everyone makes a suggestion about how THEY can help. No accusations about others. Everyone listens as each person speaks, no interrupting.
Resolve to get counseling / ask your GP about family therapy.
When you are desperate, do not see ex as a way out.
Do not use the children and their care as a means to get back at him. Having the care of your kids is not revenge!
You cannot use your 14 yo as a human shield for your 12 yo. It is not her responsibility. She wanted to come home.
Look after yourself: therapy / counseling.
Take control, keep control. It will give you strength and strength will give you calm.
You had the arrangement. Text calmly and say you are keeping the arrangement, see them at 1pm.