Many minor and tribal languages are not available in school curriculum and the number of speakers is receding in those languages. What is this known as in socio educational linguistics?

When the number of speakers of a language is declining and it is at risk of disappearing, it is termed language endangerment. Language death refers to the complete extinction when the last speaker dies. Language minority in education refers to status, not the process of decline. Language revival is the opposite — efforts to bring back a language. The process described (receding speakers, absence from curriculum) is endangerment.