Mass attendance works best when it is introduced gradually. Reading the social story "What do we do at church?" (see the page on Sacramental Preparation) before attending Mass each week helps.
The key is to make Mass attendance a positive experience. What frequently works is Initially attending Mass for only a few minutes at a time, gradually extending the time, and getting a treat afterward. (Building up to the full Mass may take two or three years.) Depending on the student, one may want to start with the first few minutes of Mass and work forward, or the last few minutes of Mass and work backward.
Consider bringing toys (but not ones that make noise), giving stickers at intervals for sitting quietly, and picking a seat that will allow for taking breaks, making a quick exit, and minimizing the student's ability to reach those outside the immediate family.
Students generally are better behaved when they have a better idea of what is going on. The Saint Mary's Curriculum uses a Mass book, which is a step-by-step guide to the Mass. Mayer-Johnson Picture Communication Symbols, original drawings, and photos illustrate the Mass, including many of the verbal responses.