Are Swedish Kroner good anywhere?

Kroner are good everywhere in Sweden :)

They're the currency Sweden uses so your son hasn't been cheated. If you pay in foreign currency anywhere you will probably receive your change in local money.

They almost certainly won't be accepted outside of Sweden (Maybe just across the bridge into Denmark?) but any currency exchange will change them.

Swedish Kroner are the currency of Sweden. They do not use the Euro.
Your on was lucky to find a shop that accepted Euros, he probably got a very bad exchange rate. Yes, you can exchange them, or save them for your next trip, but very few currencies exchanges accept coins, notes only.

Shame on the school trip organizers for not informing the students about Swedish currency.

Thank you all so much! I feel like an idiot. The school told us to send euros with our kids (probably because they are easier to get ahold of than kroner), so I assumed that Sweden was on the euro. I should have consulted google rather than troubling you all. Thank you!

The trip organizers would benefit from visiting this website.

What a terrible piece of advise from your sons School. Knowing the currency should be foreign travel 101 for everyone. By exchanging Euros for your son, only to have him change them to Swedish kroner or getting a bad rate by using them in a shop, you have paid a conversion fee twice for no reason.
Also if the School had done it's homework, they would know that Sweden is an almost cash less society by now, cards and increasingly apps, are the way to go.

Scandinavia is mostly cash free. You are not given a second glance if you buy a pack of gum with a credit card (or in my case, a role of something like lifesavers.)

So many "school trips" are poorly organized and managed by school leaders who are only leaders in name - usually they collect the money and send it on to EF Tours who does the rest.

Sad that the school missed a teaching moment. Of the five Nordic countries, only Finland uses Euros (EUR). The four Scandinavian countries each have their own version of Crowns:

Norwegian Krone NOK
Danish Kroner DKK
Swedish Krona SEK
Icelandic Krona IKK

For those needing the comfort of having local currency in hand before departing North America, a Nordic tour would require five different currencies.

Whether you want to give your son a credit card is up to you. But truly, Scandinavians (Swedes) use credit cards for everything. I used my card to pay 25 cents to use the toilet. Every small vendor is set up to use cards.