As if often the case, the conventional wisdom is wrong. Individuals with lower levels of education actually attend church less frequently than those with a high school education or less. As the table below indicates (from the 2008 General Social Surveys accessed using the ARDA's "Quick Stats" feature), 22.7% of individuals who didn't finish high school and 22.6% of those who only completed high school report that they never attend church (both above the national average), while only 17.6% of those with a bachelor degree and 18.8% of those with a graduate degree say they never attend church. The opposite is also true. Those with bachelor and graduate degrees are more likely to report that they attend church weekly (24.3% and 22.6% respectively) than those with a high school diploma or less (17.0%). Some college appears to have some interesting effects. Junior college graduates are the least likely group (15.3%) to never attend church and the most likely group to attend church more than once a week (11.0%).
The bottom line is that higher levels of education do not predict irreligiousness (or rather, church attendance). Education levels do, however, predict the "type" of religion people are attracted to (if in fact they are attracted to any at all). Individuals with higher levels of education tend to prefer faith communities that have somewhat accommodated their beliefs and practices to the wider society (i.e., they have become somewhat secularized) whereas individuals with lower levels tend to prefer faith communities that resist the trappings of modern life (i.e., they are far less secularized). The former are sometimes referred to as "church-type" churches and the latter as "sectarian" churches. It is probably best to see this more as a continuum than as a dichotomy, running from highly sectarian churches at one end to highly secularized churches at the other.
What are some of the more reliable predictors of frequent church attendance (or non-attendance)? Well, females tend to be more religious than males and as such attend church more frequently, and at least in the US, non-hispanic whites are the least religious race/ethnic group and so attend church less frequently. Another strong predictor is geographic mobility. People who move around on average don't attend church as often as those who have lived in a community for a long time -- probably because people are less likely to invest the time in a faith community until they know that such an investment will yield tangible benefits.