Perhaps the most encompassing constitutional language, and maybe the best model, comes from Montana, which does not presume the right to education is limited to children.  However, none of them has committed to a right to a free higher education.

Montana

Section 1. Educational goals and duties. (1) It is the goal of the people to establish a system of education which will develop the full educational potential of each person. Equality of educational opportunity is guaranteed to each person of the state.

The Illinois state constitution is high-minded but includes strong limits:

Illinois 

SECTION 1.  GOAL – FREE SCHOOLS

A fundamental goal of the People of the State is the educational development of all persons to the limits of their capacities.

The State shall provide for an efficient system of high quality public educational institutions and services.  Education in public schools through the secondary level shall be free. There may be such other free education as the General Assembly provides by law.

The State has the primary responsibility for financing the system of public education.