Many of our age-related expectations for children, such as when they will read, write, sit at a desk, or even pay attention at circle time, are rather arbitrary. While these things are important, we ought to think of them as branches of a developmental tree and first be sure there's a sturdy trunk providing support.
Looking at Developmental Stages
In order to meet ordinary preschool expectations, a child has to have acquired the ability to focus and attend to another person, and to engage easily and warmly by exchanging simple friendly gestures, smiles, emotional expressions, and meaningful sounds. She must be able to communicate, to show what she wants, and take the next steps toward logical thought and problem-solving.
Children with special needs are often missing some pieces related to these prior stages of development. Does the little girl in your class express her desires verbally or take you by the hand to indicate her wants and needs with gestures, sounds, and the like? Unless one can relate, engage others, and gesture, it's not possible to go to the next steps of formal learning. But when that foundation, that trunk, is solidly there, we can almost always help a child learn to read, write, do math, and live a "normal life."
So the first thing to do for this little girl who has special needs is establish where she is on the developmental ladder. Her progress may be slow because people are not tuning in to wherever she is. Perhaps she requires some practice in simple gesturing and relating or in doing some preverbal problem-solving.
All children are capable of making some progress, going from their level to the next, but first we have to know where they are. This means not focusing on how far behind they may be. So, for example, if this child is not taking an adult by the hand and showing what she wants or using words and sentences to describe her needs, that's the level you should be working on, rather than pre-reading or writing activities, matching picture cards or shapes.
Children who don't have special needs effortlessly learn to attend and engage, acquire shared language, and spontaneously use ideas creatively and logically. With these children we can get right to the focus on pre-reading, writing, and math, because the foundations have been learned without our paying much attention to them. We have to work at these foundation pieces, the building blocks of all learning, when a child has special needs.
Mastering Current Milestones
With this little girl, your task is to help her to be engaged and interactive all the time. Rather than asking her to simply join group activities, you or an aide might do more individual work with her. Look at books together, maybe encouraging her to point to the picture she likes and then perhaps to use a word to describe what it is she likes about it. You might choose to have long discussions about a snack, whether she wants the big glass or the little glass of juice. And then you or the aide will want to join her at pretend play to get her language going where your dolly talks to her dolly.
You can also take little everyday challenges like opening a door and raise questions, such as "Who's going to turn that doorknob?" Be very expressive: "Oh, I can't do it. It's hard." Any little interactions like these will help her be interactive, use ideas logically, and eventually bring her closer to reading, writing, and math.
It may be hard to believe but helping her master her own current milestones is the surest way to achieve the goals you and her parents have for her.
Assessing Pace and Progress
As for the rate of her progress, that will depend on two factors: her natural ability and whether, as we've been saying, she is offered the opportunity to practice what she currently needs to learn.
Typically a child approached where she is will move ahead, although she may plateau and then pick up again. Even in an optimal learning environment, a child will often move ahead quickly, then seem to level off for a while. The child's progress is still continuing but in a more subtle way. Soon you should see more spurts of growth.
So, to answer the questions about how far she can go, consider what has happened over several months. That should tell you, too, if her learning program is correct. No progress over a number of months means you have to reevaluate the program situation and also make sure there isn't something else going on medically or in the family, or whether there are just temporary shifts in this child's own nervous system due to growth and development. And remember, even a cold or an ear infection can cause a temporary slack. The point is to worry less about her day-to-day accomplishments than about whether the child is going in the direction you want to see.
We should look at every special needs child's learning curve over a period of time. If we see what has happened in an optimal program, between ages two and four, for example, we can get some sense of where that child is going to land. And even then, the journey is really the best determiner of its own destination. So watching the learning curve over a period of a few years can give us a pretty good idea of whether this child is moving in a way that will lead to certain skills being developed in time.
Right now, though, you can advise the parents not to worry about their child catching up. All of you should focus instead on building the trunk of skills that ordinarily are learned early in life.