Need help calculating AVG scores

[Bill Shroka]

Hello,
I use the following query to determine the average score by teams taking a particular test.

SELECT teams.teamname, teams.id, AVG(game_sessions.score) as score
FROM teams, game_sessions
WHERE teams.id = game_sessions.team_id
GROUP BY teams.teamname
ORDER BY score DESC LIMIT 0,25

What I'd like to change now is instead of averaging all of the scores from each team I'd like to only average the top 50% of scores from each team.

Is this possible?

Thanks.