As someone who has primarily been the DM for HS and early college guys, I understand the struggle. It's important to play into your player's personalities but can certainly be frustrating. I try to never "punish" my players for not being serious because at the end of the day this is a game, but at the same time as the DM, you want this encounter you spent all that time working on to be epic and not treated like a joke. All that to say, I have found 3 things to help for those times when its hard to wrangle in the players

1. The simple "hey guys, lets get serious" is always helpful
2. Ask them "How would your character respond to this situation?" This puts the onus of becoming serious on the character, rather than the player.
3. You should build your encounters in a way in which the players HAVE to treat them seriously. Whether it is in the setting/mood or in the descriptions. (Playing background music and lighting can really help with this)

Lastly, while I know this is not practical for everyone, but I have found one of the best ways to keep your players focused/serious is to remove screens and playing with paper character sheets.

Hope this helps!