Almost half the staff is a Texas certified teacher. or Almost half the staff are Texas certified teachers.
You need "are" in that sentence.
The correct sentence is "Almost half the staff is a Texas certified teacher."
In this sentence, the subject is "half the staff." Since "half" refers to a singular quantity, we use the singular verb form "is." The phrase "a Texas certified teacher" acts as a modifier that describes the type of person being referred to.
If you wanted to use the plural form, the sentence would be "Almost half the staff are Texas certified teachers." Here, the plural verb form "are" is used because "teachers" is a plural noun.
To determine whether to use a singular or plural verb form in these types of sentences, it is important to consider the quantity or number of the subject being referred to.