My high school students can't read

I teach 10-12 graders in a variety of required courses and electives. I teach in Massachusetts, which is known for its high quality education and I'm in a fairly well regarded school district. A number of my classes are AP or honors level, but it doesn't matter the grade or rigor level, so many of my students don't know how to read. They can recognize familiar words, but I notice when I ask for volunteers to read aloud when they get to an unfamiliar word they either pause and wait for me to tell them what it is, or "guess" a word that doesn't make sense in context. I'm wondering if this is the result of these kids learning to "read" using the Lucy Culkins method (which my district did use when they were in elementary school)? And more pressing, I'm stumped for how to help them learn to sound out words. I am well versed in teaching them to source a historical document, corroborate and contextualize texts, etc. but I'm not a literacy teacher. I'm tempted to say "sound it out" like my kindergarten teacher would have done to me, but I'm not trying to embarrass them further. It really gets in the way of their deeper understanding of documents. Anyone else seeing this and/or have tips?