1. Every pupil is a school will have to abide by the discipline and rules of school including attending prayer.
2. Since this is Christian school you can no expect them to tech your child hindu hyms. It is quite natural that during assembly they would attend christian prayers.
3. Now if they practice or propagate religious strictures or sermons then it is prohibited.
4. Otherwise a daily christian prayer is very much permissible and if you have any objection you can get your child admitted to any other school.
5. i do not think you should interpret their actions under orthodox religious colour.