As another poster said, it really comes down to how much you want to spend. The good news is that there are understated pens in pretty much every price category. So many pens come in a classic, relatively undifferentiated cigar shape, and almost always with a black colour option, you really are spoiled for choice.
Were it me, I'd go for one of the minimalistic Platinums, Pilots, or Sailors. Reason: Lots of nib options. But this presumes you're willing to be in that price range -- which, depending on model, isn't going to be too far off from the Lamy 2k.
Despite the very strong bank of suggestions provided to you, here, I suspect/speculate a couple of things:
1. You may want something at a lower price point. I'm basing this on your current pens. But, then, the Lotus you asked about is getting up close to the range of the 2k, so I'm happy to be wrong. If you tell us how much you want to spend, this very kind cabal will zero in on suggestions that hit your wallet just that hard.
2. You may not want to deal with a vintage pen, no matter how reliable, both because it's more difficult to get one in Brazil, and you may not want to use a vintage pen in a school environment. This is all entirely reasonable, but if you're intrigued about vintage pens, there are some that might still be worth considering. I'm thinking of the Parker 51, which is a legendary workhorse, especially the aerometric version, and the epitome of understated style. (SoulSamarai posted a pic, above.)