The expectation by many people is that the hearing aids will correct hearing as glasses do vision, but this is not the case. Hearing aids provide amplification of sound that is very helpful. But there are other problems some people who are hard of hearing experience, such as not being able to discriminate the speech sounds they can hear, even with the added volume, and not being able to isolate speech from background noise. Hearing aids alone do not correct these problems. As a result, their ability to understand speech varies greatly, as do their needs in the classroom.
Assistive technology can stretch a hearing aid's capability.
Even so, with hearing aids and assistive listening devices, students who are hard of hearing will still be working hard to hear. It's probably safe to say they can never relax and just absorb what the teacher is saying. They have to go through an additional step of listening intently to process what they are hearing and then form a reaction to it. The listening and processing part alone takes a lot of energy. In addition to hearing aids and assistive technology, they will be using speechreading and any other strategy they know to help them understand speech.