What's the biggest problem? Holding the place value with a zero. I've found that when children are asked to "build" their own numbers, they immediately understand that something's missing when a place in the middle of the number is empty. You can do this in a game-like activity with cards or dice, or it works just fine with worksheets or the teacher calling off the numbers for students to fill in. Just tell them which digit to write in which place - - - but don't fill every place.
In my classroom, we use the term "collapsed" to describe a number with a missing zero. Kids can now see that each period needs three digits, each in its proper place. If you do this step before asking your students to write numbers in standard form from written or oral words, things will go much more smoothly in your classroom. Soon your students will understand that when you say "two hundred three thousand," they write 203,000 (instead of 23 or 23,000). Try it! You'll like it!