Group students into pairs and have each pair get a basketball and go to a hoop. For an instant activity, have the students play one on one basketball to get warmed up. Don't worry much about all the rules, just make sure they are safe.

After the warm-up, call them in for a discussion. Discuss the fact that basketball can be a complex game and that there are many rules and strategies to playing. Tell them that one important concept is defensive positioning and that today's activity will stress defensive positioning when you are playing person to person defense (you may need to explain what that means). Tell them that they will go back to their court and continue playing one on one, but this time the whistle will sound every so often. They must freeze when the whistle sounds. This is just a practice game to get them used to freezing when they hear the whistle and that the focus is on freezing, not defensive positioning yet.

As soon as the students are good at freezing, move on to the real game. Bring the students in and discuss defensive positioning. Tell them that the main concept for today will be that the defensive person is closer to the basket than the offensive person. Demonstrate this concept. Tell them that they will not keep score in the usual way. They will keep score according to who is in better position when the whistle sounds and they freeze. Students will continue to play one on one like they have been playing. When the whistle sounds, if the defensive person is closer to the basket, he/she gets a point. If the offensive person is closer, he/she gets a point. Also, points are earned every time a basket is made. This encourages students to play without camping out underneath the hoop. Have students play and blow the whistle every 30-50 seconds. Give them time to figure out who got the point then tell them to continue playing.

Try the same concept with 2 vs. 2 at one hoop.
Include the concept of defenders staying between the player and the basket.
Address concepts related to slowing down offensive players by body positioning, or causing them to run around defenders.
The same concept can be applied in hockey and soccer.

Have a third person in a group conduct a peer check every time the whistle blows. Provide a check sheet that has two columns, a "yes" and "no" column. When the whistle blows, the student makes a check mark in the appropriate column indicating whether the defensive player was in correct position.
Use an exit slip that has a number of hoops drawn with "O's" written to indicate where the offensive player is. The students must write a "D" showing the appropriate positioning for the defensive player.