Teachers used harder fonts on their slides, and handouts. Later they photocopied hand written work and blurred a bit. The teachers then used on class as the control group, giving them the perfect and not hard to read copies.
The other class were given harder reading material:
-12-point Comic Sans
The big finding was that students with harder font answered 86.5% of questions correctly.
The other group, with the clear text scored 72.8% on the same test.
This is because we use our brains a little more on hard material, making us have a better understanding.
Diemand-Yauman, C., Oppenheimer, D., and Vaughan, E. (2011). Fortune favors the Bold (and the Italicized): Effects of disfluency on educational outcomes. Cognition, 118 (1), 111-115 DOI: 10.1016/j.cognition.2010.09.012