When it comes to in-season training there are a few important principles to keep in mind to ensure that you're creating the best program that will allow your athletes to be successful in competition. While traditionally periodization models have been built around single competition sports such as weightlifting or track and field, team sports are unique due to there long seasons and having competitions each week and possibly multiple times a week. So how do we reconcile this with physical training? The answer may not be to use a traditional periodization approach, but rather one that looks at the schedule and adapts to the needs of the athletes.
First it's important to point out that the overall goal is to manage stress and keep and athlete healthy. At no point should adding unnecessary volume or chasing numbers be the goal or the direction taken when planning out an in-season program. Important in this is that it's not just training stress, it's stress from competition, classes if they're in school still, from their spouses, and also just general life stress. All of this must taken into account and be managed to ensure the athlete is in the best place for competition.
Piggybacking off of this, you'll want to limit exercise variation and not put anything new in the plan here. New movements and exercises tend to cause soreness and that's the last thing you want during the season.
It's important to realize that while commonly done, there is no "maintenance" phase, you are either getting better or worse, so when putting together your plan be sure to not fall into the trap of wanting to be conservative with intensity for fear of injury, as this may actually increase their injury risk because they will lose strength throughout the season. And with many team sports having relatively short off seasons and preseasons, utilizing the in-season to get better should be at the top of the list because this will be the longest uninterrupted block of time throughout the year. The key point is to find windows of opportunity to train hard, so obviously don't plan a max strength session right before competition, but rather by knowing how much time is necessary for supercompensation, this will allow you plan when is most effective to get your training in. This also means that you should have a working knowledge of how long your athletes can maintain their biomotor abilities (strength, speed, flexibility, endurance, technique) to ensure that you train each quality within this timeframe to maintain and improve it's capacity.
When it comes to exercise selection, you want to keep variation to a minimum, always changing exercises or doing unfamiliar work will cause soreness and this is the last thing that you want to happen during the season. Generally you should only pick exercises that are specific to the demands of the game and allow for large ranges of motion to maintain mobility throughout the season.
A final point, is to acknowledge the demands of their sport. Practices and games provide a good stimulus for biomotor abilities based on the sport, so it should be noted to only add in what is necessary on top of this. But on the other side of this is realizing that sport is not the best stimulus for fitness, so realizing where the sport adds fitness and where it falls short is the important point here.
Putting this altogether is a good starting point to plan an in-season training program, however all of this assumes that capacities have been built in the offseason and preseason and the in-season is the time to continue to progress to ensure that athlete is at their best for the entire length of their season.