We have known for a long time that more speakers in a live space (meaning a space with many hard and reflective surfaces) can leave you with not that many options for speaker installation for a PA system. There are 2 basic approaches to solving this delivery of sound, primarily the spoken word to these environments. First and typically the best way is to have a single point system which delivers the sound to an auditorium or a congregation from only one point, this type of system will have less potential for reflective sound. These systems work best if you can aim the sound directly into the ears of the listeners from above, this is why you see clusters or better said an array(2-10 speaker boxes of one model), of a two way speaker box, which creates a curve due to the box design being slimmer in the back than the front. You see this type of installation at football stadiums (professional) where there will be many of these types of arrays located all around the field. Once again they are aimed and spaced such that each group of fans hears only one cluster or array. That works very well but it is impossible for all fans to hear only one array as there are many areas where 2 sets of arrays will be heard. This is OK as long as there is no delay (or equal delay) inserted between these uniformly placed arrays. Proper delay is used in a meaningful way by allowing speaker arrays or units to be in time with each other from a center out method meaning that if there are speakers in the center of a installation and speakers further out towards the upper or lower decks or lodge areas that the sound from the most center speakers and the sound from speakers further from center would be in time with each other. Sound travels around 600 mph or around 880 feet per second so if your inner and outer speaker were 100 feet apart you would want about 1/9 of a second delay on the outer speakers. This may not seem like much but that type of distortion to the listener makes a system far less intelligible.
Getting back to smaller systems like school gymnasiums. I have worked in many schools and churches. Usually there is already a system in place which I have to work with. Generally there are still two approaches to the PA: Many speakers placed to aim at the bleachers or a single round cluster in the center of the gym, or there may be 2 or 4 speakers placed above the playing court aimed at the bleachers. In all these cases this leaves the playing court non illuminated with sound, which is a good thing for the most part because the playing court is highly reflective and having speakers placed to illuminate the court will lead to a very poor sound in the rest of the gym, due to the reflection. One very important change I have been working on for the last 10 years is to have a portable mixer on the score keepers table for the PA announcer and the dance team instructor to be able to mix their own levels while hearing the system play, which is a drastic improvement over having the equipment and levels all in an adjacent room.