This is a good question. The article you referenced offers lots of good advice about improving your writing skills. Just like with most other things (playing an instrument or a sport) you need to practice.
With that being said, managing time on a writing exam is a test taking strategy. Here are a few things you can do to help you prepare for the test.
Find out how the test is given. Some states give students a specific amount of time to plan, write and edit. This is done in one day. Other states break up the test and on one day give you a certain amount of time to plan and to begin writing. Then the next day you get time to finish writing and edit.
Find out how your writing will be graded. States have this information listed (and your teachers should know) usually in a rubric. They break down what they are looking for and how much each part is worth.
Look at samples. Once again, your state most likely has sample papers with the scoring for each explained. Look online for this information. If you have trouble finding this information, ask you language arts teacher.
The more you know about how the test is given, how it is graded, and have examined some samples, the better you can prepare for taking the test.
Now for the time management part. Simulate the test at home. Find a quiet spot where you can sit at a table with good lighting. Get a stop watch or timer, sharpened pencils, and some lined paper. Ask your language arts teacher for different writing prompts. Depending on how your state's test is administered will determine how you practice for the test. Here is an example:
Get everything ready - pencils, paper, time.
Select one of your writing prompts.
Set the timer for the planning time - depends on state guidelines.
As you plan, keep an eye on the clock. If they suggest that you have 45 minutes to plan, then take most of that time planning. Think through your ideas. Think of descriptive words that go with the topic. Having the words handy will help when you actually help you when you start writing.
If your state requires a break and to continue the process the next day, then that's how you should practice. Follow the process just like how the test will be given. Continue to set the timer. This will help you to get a "feeling" for the time when it is time for the test.
I know my response was a little long, but hopefully it has given you some more things to think about when you are preparing to take the test. If you have another question, please don't hesitate to contact me directly.