What Schools Want
In general, schools are reluctant to admit a student if the student is weaker than a minimum level for the class he/she will be joining. This weakness is usually tested in Math and Reading (sometimes in Writing) and in Cognitive Abilities.
What Is Needed
Since all schools are different, the minimum level for admission for each school is different. Schools will usually not disclose specifics of what they will test. Therefore, parents need to ensure that the student is generally proficient in key subjects (Math, Reading and/or Writing).