Convict tangs are supposed to be the easiest to get to shoal. All tangs will shoal occasionally to raid someone else's territory, usually a damsel's but sometimes a more territorial type of tang like powder blues. Banner fish will shoal, as will pyramid butterflies.
A few people are bringing in some types of small cardinals that tend to school or shoal, but there's usually a lot of die-off. These fish need to be fed almost as soon as they arrive as they are not particularly robust, and live food is usually necessary, at least until they start eating.
Dave.M