To make it comfortable for Kasper and David to attend school, they should have achieved the English Proficiency level representative for Junior High.
The Common European Framework of Reference for Languages (CEFR) differentiates the following proficiency levels. It was originally designed for adults.
It doesn’t represent the progression of a child, which can be demotivating (cite).

Proficiency levels for kids and teens - from Cezch Republich British Council
The British Council targets the following levels with their English for kids and teens courses. They also support parents in teaching their children English (booklet).
Cambridge Assessment - Help your child learn English
See Indian’ British Council site.
Learning outcomes:
See Indian’ British Council site.
Learning outcomes: