Final answer:
The most effective strategy to assist a middle-schooler struggling with 3rd-grade high-frequency words is providing additional practice with high-frequency words.
Step-by-step explanation:
The most effective strategy to assist a middle-schooler struggling with 3rd-grade high-frequency words would be providing additional practice with high-frequency words. By regularly practicing these words, the student will become more familiar and comfortable with them, which will ultimately improve their reading and comprehension skills.
While using flashcards to review high-frequency words can also be helpful, it might not address the specific issue of a struggling middle-schooler with 3rd-grade words. Reading books at a higher reading level might be too challenging for the student at this point, and encouraging the use of context clues alone might not provide enough focused practice on high-frequency words.