。
It's hard for a returnee kid to fit in with that class.
Literal
Returnee-child [subject-が] that class [inside-の中に] blend-in [nominalizer-の] [topic-は] difficult [casual-よ].
帰国子女 ('returnee child') refers to Japanese children who lived abroad and returned — they often face social friction reintegrating. The term carries cultural weight: Japan has specific school programs and even entrance exam categories for 帰国子女. 溶け込む ('to melt into, blend in') vividly captures the desired seamlessness of social integration. The のは~ construction nominalizes the difficulty.