TL;DR: school/tests/exams don't allow phones.

Here in NL - Casio FX-82NL is allowed during test/exams for middle/high school, and actually for Radio Amateur/HAM licence exam - they even hand you one of their FX-82NLs.

Other more advanced (graphing, with memory/Python/etc) are also allowed in some places, but they need to be set to exam mode that disables memory/python/etc.