Tuckable IWB holsters are easily concealed but present potential difficulties for a smooth draw.
Active-shooter incidences are extremely rare, so that scenario should not be your primary concern. Rather, consider more common (but still potentially deadly) crimes like the armed mugger in the parking lot or an angry customer who chooses to commit life-threatening violence.
I’ve carried in office environments for more than six years, and before I did so, I sought out the opinion of qualified instructors like NRA Senior Training Counselor Jim Neff of Generations Firearm Training. His advice was to keep in mind the different materials our clothes are made of and how they will substantially change your ability to access a concealed firearm. Shirt length, size and shape of pockets, location of belt loops and looseness of pant legs are different between almost all brands. The best way to demonstrate this is with your standard slacks versus your favorite jeans. Try the same methods in both, and you will usually see a substantial difference.
Neff did not forget about women who carry in the office. “The same subtle changes—in skirt lengths, clothing material, shirt lengths, pant leg looseness and shape of hips and waist—can make a big difference regarding firearm accessibility,” he says. “Off-body carry becomes more common with women. Accessibility and retention need to be seriously considered when using such methods.”
It’s important to run tests and see what options for office carry work better for you than others. In order to find out which holster works best for the task, I’ve selected a common handgun for such purposes: a Kel-Tec P3AT with a Crimson Trace Lightguard weaponlight. Neff and I ran through two different scenarios that mimic “real-life” workplace environments with the P3AT using five types of common concealed-carry holsters. We then averaged our scores together to establish a basis for comparing the effectiveness of each holster.

Off-body carry provides an excellent means of concealment, but firearm-retention becomes a serious concern with such devices.
The holsters used were:
A tuckable inside-the-waistband (IWB) holster, tucked into a dress shirt
A pocket holster on the strong side
An ankle holster
A day planner designed to carry a concealed pistol
A computer bag with a small pocket for a handgun
Scenario #1
From a seated position, hands on table. On the start signal, stand and engage a target 15 feet away with one round. Repeat three times for each holster. Day-planner holster on the table, computer bag on the ground next to the table.
Scenario #2
Standing, arms at sides. At start signal, engage a target 15 feet away with one round. Repeat three times for each holster. Day-planner holster in the support hand, computer bag on the support-side shoulder.
Results

 

A pocket holster may be the best method for carry where concealment and ease-of-draw are your primary objectives.
Based on these results, if you carry a small pistol in an office environment, consider carrying your gun in a strong-side pocket holster. However, be sure you can access it from a seated position, and spend time on the range practicing your presentation while sitting down. In addition, this test clearly showed having a hand on the pistol in advance of something bad happening saves valuable time. Even in the office, you must be aware of danger and take steps to react appropriately.