Many schools have dictionaries appropriate to the students grade level, often they are missing many words the students are questioning about. Students need access to all dictionaries because sometimes the words they read or hear are above the grade level but it is important for them to learn it at the time they query about the word.
Dictionaries are a great resource for learning new words. Incorporate dictionary skill lessons into your daily plans. There is about 200 days, that is 200 new words for a student, multiply that by 12 years of schooling. The student will have an additional 2400 words in his vocabulary.