To Gain and to Give: Hilchot Tzedakah, Day 30
It was previously noted that the minhag is to permit the use of ma’aser funds for various non-tzedakah mitzvah causes. However, ma’aser money may not be used towards any mitzvah on which one is anyway obligated to spend money. For example, one cannot use ma’aser funds for matanos la’evyonim on Purim. The same halacha applies to using ma’aser funds for tuition. Because one is required to spend money on matanos la’evyonim and on providing their children with a proper upbringing, therefore ma’aser funds cannot be used to pay off these obligations.
צדקה ומשפט פ"ו ס"ב