Describe some of the specific ways you think you can be of help, and discuss your availability and commitment. Ask what ideas the teacher, counselor, or administrator has about how you can help with astronomy and science education. Follow-up with a note and phone call. Teachers and administrators get extremely busy and may have difficulty getting back to you as soon as you would like. Take it upon yourself to make follow-up contact.
Many amateur astronomers belong to local astronomy clubs.
Each club usually has some members who are interested in education and explaining astronomy to the public. Often, astronomy clubs hold star parties for the public, or go to local schools for one-time visits. You will want to find the club members who are interested in conveying astronomy to others and have at least some experience with children. Call the club president, attend a local meeting, or show up at an evening star gazing session and talk to the members. Most likely you will find someone who is enthusiastic.