Generally, it's best to take the ERB (ISEE) in December. The ISEE is an academic-based test, the more you have learned, the better you do. You're graded against a same-grade based peer group.
From the ERB, the people who administer the ISEE:
It is important to note that the ISEE may not be taken for practice; it may be taken only for the purpose of providing scores to participating schools as part of the admissions process. An applicant may take the ISEE only once per admission season or six month window.
Tutor Anthony-James Green is currently working with students applying to NYC private school who want to improve their math, vocabulary, and writing ISEE and SSAT skills. To learn more, contact me at 212-712-2228 or eglickman(at)abacusguide.com or contact Manhattan's top ISEE tutor, Anthony-James Green, at 914-400-3859 or firstname.lastname@example.org. Good luck! www.privateschoolauthority.com