Say you have a team wanting to only play defense. Could all 11 players (1 goalkeeper plus 10 other players) literally stand in the goal box, essentially forming a wall, to prevent the other team from scoring? Could they play an entire match this way?
I understand the rule that only one player can act as goalkeeper (only they could handle the ball), but the other players would behave as normal. They could only interact with the ball by kicks, headers or other legal means.
dly's answer correctly covers whether this strategy would be legal or not. However, I'm going to argue that it doesn't matter if it's legal, it's not going to be effective.
With 11 players standing in the goal, I'm just going to get a couple of my players to run up the pitch with the ball, then when they get to somewhere about 12-18 yards away from the goal, kick the ball really, really hard somewhere towards the top corner of the goal. The chance of you keeping that out every single time is minimal - sometime, I'm going to get a ricochet off something and the ball's going to go in. Now you've got to change strategy, and you're already a goal down.
Yes and No... the rules (laws) don't prohibit this. You could basically place all 11 players on the goal line and it would go by the rules. Of course there are certain situations where you are forced to leave the box (e.g. a penalty against you), but you're not forced to play offensive.
However, the FIFA Code of Conduct says Play to Win (click):
Winning is the object of playing any game. Never set out to lose. If you do not play to win, you are cheating your opponents, deceiving those who are watching, and also fooling yourself. Never give up against stronger opponents but never relent against weaker ones. It is an insult to any opponent to play at less than full strength. Play to win, until the final whistle.
This kinda forces you to not just clear the ball and stay behind. But there is no clear rule of how to do it, so it's a grey area and you can't really draw a line between acceptable and unacceptable. It really depends on the situation.
There have been games where teams just stood behind and cleared the ball, simply because they weren't able to withstand the pressure of the opponents. That's completely fine and acceptable. From my own experience, we've saved a draw by doing this when we were two men short after two red cards. There is no point in attacking at all costs and eventually losing the game.
Unacceptable is playing to lose on purpose or doing something as pointed out in this Q&A.
