A recommendation letter can only be as strong as the raw material the teacher has to work with, and most of what determines that quality happens before they ever start writing: who you ask, how much context you give them, and one specific checkbox most students click through without understanding.
How many you actually need, and from whom
Requirements vary by school, so check each one directly rather than assuming a single standard -- but the common pattern is a required counselor letter plus one or two teacher letters, with more selective schools more often expecting two. Your counselor letter is close to universal at schools using the Common App; teacher letter counts are where the real variation shows up, so confirm the specific number for each school on your list rather than defaulting to whatever your first-choice school requires.
Which teacher to actually ask
The strongest letters come from a teacher in a core academic subject (English, math, science, social studies, or a foreign language) who taught you recently -- junior year is generally preferred over freshman or sophomore year, since it's the most recent complete picture a recommender can speak to, a point covered in more depth in our piece on the junior-year planning timeline. The instinct to ask whichever teacher gave you the highest grade, or whichever one you personally like best, isn't quite the right filter -- what actually matters is whether that teacher can speak with real, specific detail about how you think, participate, and grow in a rigorous academic setting. A teacher who remembers a specific moment you pushed through a hard concept or asked an unusually sharp question writes a fundamentally different letter than one who only remembers your grade.
Give your recommender something to work with
Even a teacher who genuinely likes you can only write as specifically as their memory allows, and most teachers are writing for a dozen or more students at once. A short document -- often called a "brag sheet" -- covering your activities, a few specific class moments or assignments you're proud of, what you're hoping to study, and anything about your background the teacher might not already know, gives them concrete material to draw from instead of generic praise. Many high schools already distribute a formal template for this through the counseling office; if yours doesn't, build a fuller version as a proper application resume first and hand your recommender a trimmed-down brag-sheet version of the same material.
The FERPA waiver checkbox -- waive it, in almost every case
When you set up a recommender in the Common App, you'll be asked to waive or retain your right, under FERPA, to ever read the letter after it's submitted. The near-universal advice from admissions offices and counselors is to waive that right. The reasoning is consistent: admissions officers give more weight to letters they can be confident are candid, and a waived letter signals exactly that -- your recommender can write freely without wondering whether you'll read it later, which in practice produces a more honest, useful letter. An unwaived request can also backfire directly: some teachers decline to write a letter at all if a student hasn't waived the right to see it, since it puts them in the position of writing for an audience of one (you) rather than for the admissions office. Keeping the right to review is rarely worth either cost.
Extra letters aren't automatically a bonus
Most schools that accept an additional, optional letter (from a coach, employer, religious leader, or research mentor) genuinely mean additional -- it should reveal something specific that your required letters, essays, and activities list don't already cover, not just add volume. A third or fourth letter that repeats the same "hardworking and kind" framing your other letters already establish doesn't strengthen your file; it can read as padding, or even as quiet doubt about whether your required letters are strong enough on their own.
What this means for you
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Pick teachers based on specificity of memory, not just grade or personal fondness. A teacher who can describe one real, detailed moment writes a stronger letter than one who can only offer general praise.
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Hand your recommender a brag sheet. Specific material to draw from is the single easiest way to raise the quality of a letter you don't get to write yourself.
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Waive your FERPA right to view the letter, in nearly every case -- it signals trust, and some recommenders won't write an unwaived letter at all.
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Only add an optional extra letter if it covers genuinely new ground. More letters saying the same thing isn't a stronger file -- it's padding.
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This is one step in the larger process, not a standalone task -- see the complete step-by-step guide to applying to college for how it fits with everything else, and where you are in the sequence.