I'm with these guys. I have a pretty strict rule never to fight with the students (yes, like you, I "play" with them, but never for long and never very hard).
Instead, I train A LOT on my own, working visualization, distancing drills, bag work, skipping rope, etc.
On a rare occasion, I call on a ringer (Bobby Spoon or acquaintence from another dojo/style) and we lock the doors. I think it's extremely important that none of your students be around when you want to crank it up a notch.
Why? Certainly not to hide your "weaknesses" should you have any. Simply, you want to train YOUR way, sweat, yell (yes, sometimes swear) and take breaks when YOU want them. You can't do this with people watching. Also, we all have a tendency to do what looks good instead of what works when there's an audience.
For what it's worth. . .