Here's my free advice- HSA diving requires a few basics- buoyancy control, task loading, and buddy awareness. These are key to having fun and safe dives.
It's usually a long, gruelling day because of all the scenarios, much like rescue class. Plan on wearing plenty of exposure protection, including hood and gloves, since you'll probably be in the water a long time. This goes for the pool sessions too.
Keep in mind that the training is much harder than actually diving with disabled divers. Each diver is different, but they usually have some level of abiliity that makes the dives easier. For example, quadraplegics might have enough motor control to equalize their ears, which reduces your task loading.
Don't get discouraged if the class seems overwhelming. Just like other scuba certs, the class is a license to learn. Afterwards, you are usually teamed up and start out with more experienced buddies, so there should be a lot of mentoring after the class as well. Diveheart works with some of the special recreation associations in the area to do some discover scuba type events at the local pools; after the HSA class, these are a great way to get more practice and refine skills. Also, the disabled participants thoroughly enjoy it.
Lastly, interacting with disabled people is sometimes a new challenge for many people. Be sure to address the person and not just their disability. Keep in mind that their disability is also part of their identity. Most of the disable divers I have worked with are usually physically disabled. I find cognitively disabled divers more challenging to work with. Regardless, diving with the handicapped is a very rewarding experience. You will get much more out of it than you can possibly put into it, and it will improve your dive skills.