Hall College Consulting recommends testing during Junior year, taking either the SAT or ACT once in the Winter/early Spring, and a second sitting in late Spring (May/June.) Some students opt to take both the SAT and ACT in the middle of Junior year, then choose whichever test they feel most comfortable with for a second sitting. There are a few colleges still recommending or requiring SAT Subject Tests, and we can work on timing for those on an individual basis.
Test prep can comprise individual study, tutoring, camps or classes, depending on how self-motivated a student is. Students who use online test prep resources can be just as prepared as those accessing test prep services; however, only if the student is dedicated to the prep. Local test prep tutors are great and accessible, and I can provide references..
Most of my clients finish testing in June of their Junior year, freeing up their summer for applications and essays, with the goal of managing applications in a lower-stress, linear process based on application deadlines,
if you didn't test during Junior year, it's not too late! Test dates for both the ACT and SAT are available from August-December, and scores are available soon after testing day. Your college can still receive scores before application deadlines.
A final word:: You are responsible for submitting test scores to colleges. This costs you $11-$16 per report. However, an increasing number of colleges are allowing students to self-report test scores and only pay to send them if admitted and attending. Check with each individual college to determine whether they allow self-reported scores, or ask me for help.
Compass Education Group offers an interesting primer for determining when standardized tests might fit into your college admissions search and application process.
https://www.compassprep.com/build-your-optimal-sat-or-act-timeline/
While my approach to testing features a hybrid of Compass's, you may find it suits your unique needs better. Let me know what you think!