What is the ruling on entering into an investment portfolio for managing investors' funds in the field of education in a European country with an Islamic bank, which gives an annual return of 20% for 7 years, without knowing what will be taught to the students?

The basic principle is the permissibility of investing in investment portfolios within Islamic banks, unless there is evidence of prohibited elements therein. What the questioner mentioned regarding a guaranteed 20% annual return for 7 years—if the intention is to guarantee the principal along with this specified percentage increase—then this is unanimously impermissible and invalidates the mudarabah (profit-sharing) contract. Instead, the return must be an undivided share of the profit, if any is realized, without guaranteeing the principal.