It's not that hard, whenever possible answer all messages negatively to reduce chances of collegues coming in, also the longer the message is visible it will also reduce your performance so do it fast.
Secondly, pay attention to what collegues say when they get in, if they say anything like "won't take long" or "I'll make it quick", you can keep working since they won't bother much, in worst case take off either the skirt or the blouse for them to be even quicker bot not both so your performance doesn't lower too much.
Some others will be harder or migth come in groups, if you see many of them or their pleasure bar isn't going too fast then quickly undress and switch to fuck.
Finally, whenever you undressed for a collegue, dress up before they're done to save a little time from what she takes putting it on