I have been asked this by a lot of coaches of younger teams especially U13. You cannot coach speed, and that means you must change your strategies. Here are a few changes to take into consideration. Keep in mind that no two players are identical, nor are any two teams identical. What normally works well with one may not benefit the other.
Now that you are playing 11v11 you're dealing with a much bigger pitch than 8v8. Seems apparent, but many coaches don't make the proper changes. From a defensive point of view, you have much more area to cover, so distribution of the work load is critical. We are all aware there are some teams that are extremely physical, technical or have a combination of both attributes. This can determine how you play without the ball.
You can look at playing deeper or dropping off whenever you lose possession to ensure that there's almost no space in behind you to exploit. This means focusing on transition in practice, because you need a minimum of 9 players back behind the ball when defending teams with pace. This allows you to deny passing lanes and as a result slow the attack.
Another tactic is to get your adversaries to play negative or square passes by way of the midfield players endeavours to close them down. A five man midfield is the most effective way to get this done. The outside midfielders should close the opponent's outside backs, when they approach the half line, to deny service. The reasoning guiding this is that teams with pace usually aim to play over the top. To prevent easy access to speedy forwards utilize screens in the midfield.
If you have an extremely superior group tactically, you can decide on an off side-trap. I personally do not like it, because I think that players at this age should rely on principles of individual and small group defending before being taught to depend on a system to do the defending for them.
After all is said and done your objective should be to train kids how to play soccer. Physical speed will come at a later point as they age, it is crucial that you understand this and do not lose sight of your long term objectives.