First, I'm sorry this is happening to another kid. Yes, it fits the definition of bullying. As far as whether a letter would help the school see the situation, I am less certain. However sometimes it is important that our children see us try to resolve the issues which are important to them. Two thoughts prior to proceeding:

1) Some institutions may boast of decreased incidents of bullying when in fact they expend their effort not on resolving such situations in a manner which decreases repeated or future aggressive behavior, but may rather focus their effort on reducing the reporting of bullying. For example, the school may shame the parent or child who brings forward an issue, with statements to the effect that the parent/child is taking up too much time, other students need to be served, tasks must be prioritized. Essentially trivializing the concerns, treating them as petty, and calling the reporter's judgment into question.*

2) You may wish to do some research and gather facts about what bullying is... and isn't... so that these facts may be incorporated into your letter, related to the facts of the incidents as they occurred. Some research ideas:
- Does your school have a bully policy (possibly found on the school website)?
- American Psychological Association (APA) has information on its website about bullying. Notice that bullying involves a real or perceived "imbalance of power". *This may be as simple as "You're the only person who cares about XXX", where XXX may be bugs, name-calling, etc. This type of statement, meant to dissuade a person from their view and coerce them to adopt group-think, may be socially isolating, demonstrating an imbalance of power.
- There's an Elementary School Counseling website which has information about bullying, coincidently one of the activities listed is called bully bugs.

Here's hoping all goes well.