"...should I choose an aggressive growth fund or be conservative?"
You should approach this as you would your own retirement savings.
Considering that the funds will be needed very soon, it is best to select the ultra-conservative option where you have some guarantee that it will not go down in value. The primary reason is that should your investment selection go down in value in the near-term, you will very likely not make it back by the time the funds are needed. Additionally, as you are likely aware, the stock market is sitting at all-time highs, over 7 years into the current bull market, and there could be a major correction downward at any time. You need to ask yourself if you are willing to accept a loss and have to pay more?
When you are so close to needing the money, you don't need to be gambling to make a few extra percentage points on the upside with the potential of losing. Again, the timeline is so short that you do not have the benefit of lots of time for things to recover.
Assuming your state offers tax benefits for contributions to its own 529 plan, you should take advantage of it and make the contribution...and again, the most conservative investment option they offer - even if that's a money market fund earning 0.01%. If your state does not offer any tax benefit for 529 contributions, I would say to skip it altogether and simply set the money aside in taxable accounts. You could put the money into CDs which mature in the year the money will be needed - 1 year to 3 year rates are currently in the 1% to 1.5% range. It's a piddly amount, but it beats your bank checking/savings account rate, you're guaranteed not to lose money, and putting the money into CDs will provide protection from the temptation to spend it.