Flanders, the northern, Dutch-speaking part of Belgium, is experiencing growing intra- and interlingual diversity. On the intralingual level, Tussentaal ("in-between-language") has emerged as a cluster of intermediate varieties between the Flemish dialects and Standard Dutch, gradually becoming "the" colloquial language. At the same time, Flanders is encountering increasing numbers of immigrants and their languages. This paper analyses the way Flemish language-in-education policy deals with perceived problems of substandardisation and multilingualism, in order to create equal opportunities for all pupils, regardless of their native language or social background. Both the policy and the measures it proposes are strongly influenced by different, yet intertwined ideologies of standardisation and monolingualism. By propagating Standard Dutch as the only acceptable language and by denying all forms of language diversity, Flemish language-in-education policy not only fails to create equal opportunities, but also reinforces ideologies that maintain inequality. Instead, language policy should be open towards language diversity, taking the role of teachers in forming and implementing policies into consideration.