St Mary school is up to tenth class only. "They don't have eleventh and twelfth class" or "There aren't tenth and twelfth class".
It's probably "St Mary's".
"St Mary's school goes up to 10th class only. They don't have an11th or 12th class."
Neither "grade" nor "standard" is used in the UK. These days, kids are in "Year + number". As far as I can work out, it starts at age 5 in Year 1 and works up from there. So, for example, a child in Year 8 is 13 years old (5 + 8).
(When I went to school, there were very different ways of expressing it but I've learnt from experience on this forum that it's really not worth trying to explain!)