I sent this to someone I thought would have a good educated answer. She replied to me thru email so I am going to cut and paste
Her responce
First of all, many deaf people don't improve their comprehension of English anywhere at all. This may be due to many factors. One is the critical period of language learning, which is believed to be from ages 0-5. This is when children learn language and internalize its rules and syntax. Once this critical period passes, it is difficult to ever become fluent in a language (of course, there are exceptions). It is very important that the child be exposed to a full, complete language - not a code, not isolated words or fragments of sentences.
Unfortunately, that is what happens all too often when a deaf child is born into a family unable to communicate with the child. If the deaf child is unable to understand spoken English with auditory aids, he or she will not have full exposure to English. If the child's parents try a mix of speachreading - impossible for a young child - and "home" signs and gestures, or even basic stabs at ASL, the child isn't getting exposure to a full language (ASL) either. That is why d/hoh children with good hearing and children born to deaf parents usually have higher reading levels - they have had this language exposure from birth.
Now, once the critical period is past, it's pretty much too late for true fluency to develop. I'm talking about first language development, not second language. Research shows that second languages can be learned and internalized at later ages if the person has a full first language - in deaf children's cases, it can be either English or ASL. But if this first language isn't developed, he/she can stay at the school until age 22 and still not read/write English fluently. In many ways, it's beyond the school's control - the parents have the biggest impact on the child's language development.
In some residential schools, however, the problem may also be with the school. It's hard for schools to find good teachers who are fluent in ASL if they're hearing. If the teachers don't sign well either, even students below the age of 5 never get a reliable adult language model, either at school or at home. Some res. schools really do not have good teachers. Many other res. schools are good, but they are dumping grounds; very often, students don't go to residential schools until they are at least 10 or 11 and have already "failed" in public schools, "failed" their parents' vain attempts to make them "normal." By the time they get to a school for the deaf, it is too late.
I'll be honest and say some teachers are terrible. Some may sign well but not teach well. Some may be great teachers but not communicate well. There are too few who can do well. But in any case the res. school is still better than the public school for many kids because there are other language models, such as peers, classroom aides, and very often the staff will be deaf (counselors, dorm staff, etc.).
Another problem is that students don't get continuous exposure to English (that is changing, with closed-captioned TV, AIM, pagers, Internet use, etc - but use of slang on AIM doesn't really help). English class at a res school isn't enough to develop fluency in English.
Now, one thing you must remember: Many hearing people have terrible English. I have a hearing secretary with a bachelor's degree, and she simply does not write grammatically correct sentences. She doesn't understand the e-mail I send her, even when I simplify it. Her manners are wonderful, she communicates very well verbally, and she does her work promptly, but written English just isn't something she understands.
Another thing to remember: With new medical "lifesaving" techniques, many babies are being saved that would have died before. They're deaf, with many additional disabilities. Today, many deaf babies are born with crack addiction, FAS, or a wealth of other problems. Those lead to learning disabilities that any child would struggle with, and present new issues for residential schools to face: How to allocate resources to a small number of "plain deaf" kids and a rapidly growing population of deaf kids with multiple disabilities, mental health issues, etc.
All in all, there really are just too many factors to answer the question simply.