Croatian Vacation

Croatia is awesome, but if I wanted a "relaxing beach vacation" it is not the first place that comes to mind. (Hawaii is. Or in Europe, maybe the Italian or French Riviera. Or Portugal.) Croatia is not a place with lots of great sandy beaches. There are beaches for sure - but it's simply not a great "beach country."

But if must go: to get to Pula, flying to Venice might be easier from the US. You can take ferry to Croatian Istria from Venice, maybe directly to Pula. Not sure how old you are, but renting a car might be the best way to get down to Dubrovnik if you are old enough to rent one. There are buses but they take a long time. I'm not sure the coastal ferries that used to run up and down the coast are going anymore.

Try to stop at Plitvice Lakes National Park on the way south from Pula - a highlight of Croatia for sure.

I don't know who told you Pula has no beaches, but they are wrong. We were just in Pula this past July and went to the beach. They have plenty. They also have a few resorts, look on booking.com or get a guide book. If you look at a map you can go to Rovinj and/Porec which also has beaches. There are ferries, about 2 hours or so, from Rovinj to Venice. Good Transportation around Crotia is lacking and we usually rent a car to get from the north down south. When we are 4-7 people we have hired a driver for the one way trip, less hassle then a car.

I'm going to assume you have at least googled or perused a guide book and know what Croatian beaches are like. Near Pula, look at Premantura/Cape Kamenjak, but you would need a car. The beaches are of the same quality from the north to the south, with varying amounts of rocks and pebbles and clear water.
If I wanted to combine Pula and a beach trip, I'd stick to the northern islands of Cres/Losin/Krk.
Mom and Dad may find the hill towns of Istria of interest, so you could all be happy.

Pula to Dubrovnik in 8 days is not something I would recommend. Flying to Pula is not that easy from US, as it requires three plane changes, but it is not that far from Zagreb or Venice, but then it is likely that anyone who flies into Venice will want to stay there at least a couple days, and you'll have to get back--every plan has trade-offs in terms of time.
Into Pula and out of Zadar is an itinerary that I would enjoy myself.

Not many ferries from Pula to the south--perhaps to Losinj.

If you stick with Istria, everyone loves Rovinj, which is just a bus ride away from Pula.

You are all going to have to plan this together, so assign everyone some reading and have a meeting. Then come back for more specific help,

I agree wholeheartedly with Andrew.

He never said there were no beaches. There are. But when most Americans dream of "a beach experience" they are visualizing something more like Thailand, Hawaii, or Cancun. Think of the striking beach image photos you see on computer screen savers...that's what Americans think of when they dream of a beach: they expect a broad, deserted, swath of sugar-fine white sand, preferably lined with palm trees, with oodles of room to stretch out, warm clear water (and if you're a high-schoool age male, probably a group of smiling, bikini-clad, young women nearby, handing you a frosty beer). I think it's foolish to plan a European vacation around this visualized dream, because you're not going to find it there. Yes, there are "nice" beaches in Croatia and in Europe, but they do not look like that dream beach. They're usually pebbley and/or a mix of coarse, darker sand. They are generally very crowded during summers, often with rows of pay-by-the-hour lounge chairs lined up cheek-to-cheek. Yeah, maybe you can point to a rare exception you found (I know of one, but I'm not telling...) but for the most part, casual US tourists won't find those unicorns (they'll find something more like this.)

To be sure, there are nice beaches there (I've been to some), but if you want all that empty space, the fine white sand, etc. - and you don't care at all about other, more "European" things (history, art, culture, etc.) - then it's just crazy to go to Europe. Go to Florida, Texas, Hawaii, even Thailand or yes, Cancun. That's where you will find that American Dream Beach - with few boring museums, paintings or old buildings. Not everyone loves Europe, and that's OK. To each their own.

You have 8 days. That suggests 2 or 3 places. You want beaches.

There is a nice beach in Split. There is a nice beach in Cavtat near Dubrovnik. Both are sandy.

I'd start in Split - stay 3 nights. Do the beach. There is a small island off shore that is a party place. Then I'd take the cat ferry to Dubrovnik and stay 4 nights. That's the trip.

You can do day trips. From Split, Trogir and the Roman ruins at Salona. From Dubrovnik, Kotor. Both towns are great.

Can you get away from the 'rents? Will they let you do stuff like drink and talk to women? I'd try to get away from them some.

Get the Rick Steves Croatia book and spend some time looking at places and choices. You need to put more time into the planning.

Of all the things to see, the Pula amphitheater is an odd choice. It’s not exactly a highlight and I find Pula to be a quite unattractive city compared to others in the area. Been there many times. What is the attraction?

Of all the things to see, the Pula amphitheater is an odd choice

Tastes vary. I thought the amphitheater in Pula was spectacular - in fact, much better than the Coliseum in Rome (because it wasn't overflowing with tourists).

We were just in Pula last week (as well as Piran, Rovinj, Korcula, Split, and Dubrovnik) We LOVED Pula . . . I agree with the former writer about the beauty and accessibility of the Pula coliseum. If you read/study RS Croatia book, you'll see there are nearby beautiful beachs, however rocky, not necessarily sandy. Also we used buses, GoOpti (a and inexpensive vanpool optionhttps://www.goopti.com/en/ ), and we flew to Split for more more Roman ruins. I think you have the potential for a great graduation trip . . . oh, and don't give up on finding the floor mosaic in the Pula gravel parking lot . . .it's very worth the little scavenger hunt.

People who visit towns for a very short time barely scratch the surface and have no real knowledge to pass on to would be visitors.