This info is also in the faqs section of the 411 site, but it never hurts to read it again. This is the very basics of the process and is super important to know.
NYC high schools are separated into 2 groups: specialized and main round. Admissions for specialized schools are completely different than main round. LaGuardia, part of the specialized group, is audition based. Admissions for the other specialized schools (Bronx Science, Brooklyn Latin, Brooklyn Tech, HS for Math, Science and Engineering, Lehman, Queens HS for the Sciences, Staten Island Tech and Stuyvesant) is based on one test only. GPAs, recommendations, transcripts, absence and latenesses don't matter. The SHSAT - specialized high school admissions test, which is held in October of 8th grade, is comprised of verbal and math questions. Cut off scores vary every year there is no finite number which guarantees admittance to any particular school. Students can take the test again in their sophomore year and transfer.
Main round schools consider GPAs, transcripts, ELA and math scores and absence and latenesses. Most do not want letters of recommendation. Some do have additional testing, interviews or submission of work.
Students have to apply to main round schools and can also apply to specialized schools but may choose not to. These lists are different and are submitted at different times. Rankings for specialized schools are done, by students, at the specialized tests. That came as a surprise to many parents. Rankings for main round schools are submitted in December. So, kids can possibly get into 3 schools: LaGuardia, a main round, and a specialized. In February students get one nondescript letter listing schools they've been accepted at. After acceptance letters are sent out many school host open houses for kids they've made offers to, to help them in their final selection process.
Touring for all schools happens beginning in the spring and throughout the fall so it's important to remember all schools are not the same. If you love Bard and Bronx Science, great - they go on separate lists. Same for Nest and Tech. But, if you like Beacon and Baruch those are both main round and will be on the same list.
Students can apply to any school in all 5 boroughs. There are over 400 with over 700 programs. There is also no zoned or safety school in Manhattan. Generally guidance counselors meet with parents in the spring to give an overview of how things work and to also give out copies of high school handbooks. These list every single school in the city along with info about graduation rates, extra curricular activities etc.