Wait a second, let’s be dan l’kaf zechut here. There are a few different ways to look at the situation. Since you were only passing by and saw through the window some boys whom you don’t know personally, there could be a lot more that you don’t know that would explain what you saw. Here are two possibilities: (Obviously I have taken two extremes and there are infinite in-between possibilities, but this should give the general idea)
1. They are under the age of 21 and are in the bar illegally. They attend yeshiva because their parents force them to but do not feel a real connection to Judaism; therefore, on this particular night, they have chosen to blow off classes and have left the yeshiva without asking permission from the rosh yeshiva. They are in the bar with the express intention of getting drunk and their conversation revolves around women.
2. They are over the age of 21 and are in the bar legally. They are respectable bochurim from good family backgrounds and are an asset to their yeshiva. On this particular night, one of them has just come back from a shidduch date with the news that he is engaged to be married. Overjoyed for him, his friends decide to go into the bar and make a l’chayim. They have double-checked the brand name of the alcohol with their Rav to ensure that it is kosher, and they are not missing any classes by sitting in the bar.
No doubt you will say that the second situation is highly unlikely. However, so is the first one, and it is our responsibility to assume the second situation since we really do not have very many facts.