According to the Purdue OWL website, parallel structure is
using the same pattern of words to show that two or more ideas have the same level of importance. This can happen at the word, phrase, or clause level. The usual way to join parallel structures is with the use of coordinating conjunctions such as "and" or "or."
For example, the following sentence has an incorrect parallel structure:
I want to go walking, bike, and swim.
In order to correct the sentence, the verbs all need to follow in suit. To correct the sentence, all of the verbs need to be "ing" verbs. Therefore, the sentence would look like this:
I want to go walking, biking, and swimming.
In order to correct the sentence in question, a few things need to be changed.
First, the sentence's structure is saying that school uniforms are not preferable because of three things: the viewpoint of students, ineffective, and costly. In this form, the sentence does not make sense structurally.
What needs to be changed is the order of the sentence. Instead of what is written, the sentence needs to read:
From the viewpoint of students, school uniforms are not preferred because of their ineffectiveness and costliness.
By introducing the viewpoint first, there is no question whose views are being noted. The next section of the sentence discusses what is being considered. The last part of the sentence explains why students do not prefer school uniforms.
By using the words ineffectiveness and costliness, the parallel structure of the verbs is appropriate. On a side note, "unpreferable" is not an accepted word. You need to change it to "not preferred."