It needs to be said that what we see on this shirt is inconsistent with the way the uniforms are portrayed in the series. A poster on Reddit mentioned that there is no way that the skirt could be tucked into the panties as the panties are not a separate piece. The entire white part of the suit is actually a single piece of clothing similar to a one piece bathing suit. This can be seen during the transformation sequences. Beyond this there is one scene which shows us otherwise unseen detail of the costumes. When Ryo Urawa has a prediction of Sailor Mercury being defeated we see her Sailor Fuku being destroyed layer by layer. This shows us that the top and bottom are in fact one piece and we also see that the breast plate is actually a hard shell over fabric.