The schools here are horrible most of the time, it is almost business. If you are saying assessment this means it is an international or private school. So, their first answer to you will be: OK, we have available vacant seat. Then they will schedule the placement assessment. But they don't have announced standards. So the results are not guaranteed (but mostly she will get accepted in a school at least, depends on what type of school she is going in Egypt and type she will go here). I advice you may apply for more than a school. The other thing is that sometimes since many people applied for many schools. So, closer to the beginning of the academic year, many schools will have vacant seat then they will start calling people who are on the waiting list but they will give priority for whom really is available for assessment.

The thing is, you will have enough time like a week or so for the assessment, but it is mostly one fixed date and no other choice.
You may arrange the visa now which will take 2-3 days. Once, they call for assessment, book your tickets which is cheap from Egypt. Still, apply in more than a school.

Good luck Mrs. Heba