It is difficult to find grade-level-specific vocabulary lists. The "Tampa Reads" site is on the first page of results of a Google search for "grade level vocabulary list standard." This site offers vocabulary lists for grades 1-5. The word "similar" appears on their list for grade 3.
Of course different educators and organizations will differ about when it is appropriate to introduce a word into students' vocabularies and on what it means to be part of the "vocabulary:" should students merely recognize the word, use it actively, understand closely related words, or also appreciate its historical roots and etymology? In these senses "similar" could be a word on somebody's first grade list and on somebody else's high-school level list.
You might get better insight by inquiring into the reading level of the context in which the word "similar" appears. A nice introduction to this topic is posted at http://www.hoagiesgifted.org/reading_levels.htm ("Reading Levels of Children's Books: How Can You Tell?").
("Tampa Reads" appears to be a private, for-profit reading improvement program, not to be confused with any public or not-for-profit agency.)
10th grade.
Grade?
Sixth or seventh grade.
I would say 4th or 5th grade.
The word "grade" can be used as both a noun and a verb. As a noun, it refers to a level or position on a scale, such as a letter grade in school. As a verb, it means to assess or evaluate the quality or performance of something.
An example of a possible selection word for 5th grade could be "discreet." Students at this grade level can explore the meaning, spelling, and usage of this word to enhance their vocabulary and writing skills.
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It was by the achievement of the whole class that we raised the grade level.
flat, grade, stage, round, floor,
No, it is not. Grade can be a verb with distinct meanings (to score, to level) or a noun with the same general concept (a score, a level or level rise). It can be a noun adjunct in terms such as grade crossing.
It depends on how you use it. Grade can mean a few things such as: third grade (noun); the grade of this product, grade A eggs (adjective), or a teacher can grade a paper (verb)
I'd say about third to fifth grade. It's an easy word to make out (for its sound), excluding the "us" part (which many students mix up for "is").