The second telephone conversation between Beatrice and Mr. Goodman in Act I of The Effects of Gamma Rays on Man-in-the-Moon Marigolds reveals quite a wealth of information, not only on Ruth, but also on her sister, Tillie.
Initially, the first conversation between Beatrice and Mr. Goodman is initiated by Mr. Goodman, and it has to do with Tillie's absences. Mr. Goodman is concerned about Tillie's care of at home. However, the reality of this conversation is that Tillie is made to stay home by Beatrice, as part of her many ways to try to manipulate her from succeeding.
The second conversation is initiated by Beatrice (p. 33-36). She is aware of the fact that Tillie is a great student for Mr. Goodman and, as a result, Mr. Goodman is working with Tillie on the Science project on the effect of the gamma rays on the Man-in-the-Moon Marigolds.
Now, let's think about what could motivate someone like Beatrice to call Mr. Goodman. First, she knows that Tillie is extremely talented. She also knows that Ruth, her other daughter, is "failing at everything". However, Mr. Goodman seems to have an interest in overseeing the success of both students. Then, why would Beatrice be interested in taking a part of things, then? The answer is simple.
It is because, in her quest to manipulate everything, Beatrice calls Goodman to cause an unnecessary issue: to inquire about the potential dangers of the project that Tillie is about to conduct.
[Tillie] told me that [the flowers] have been subjected to radioactivity and I hear such terrible things about radioactivity that I automatically associate radioactivity with sterility, and it positively horrifies me to have those seeds right here in my living room...
In reality, Beatrice could care less about the project; it is her fear of losing control of a potentially-gifted daughter what really drives her to meddle. Moreover, the fact that Ruth, who is "failing everything", has been working as a "secretary" for Mr. Goodman it is not a good thing in Beatrice's mind, either. This is because Beatrice's mind operates in a different way and she believes that Ruth should merely be excused from any work based on the fact that she is prone to seizures.
If Beatrice can find a way to control things, in order for them to operate exactly how she wants them to be, then she would be satisfied. Otherwise, she will complain.
Beatrice tells Mr. Goodman that, more than likely, Ruth is over-doing her position as the teacher's secretary to catch up with Tillie because she is "jealous". However, to accept Beatrice's point of view is to belittle, reduce, and limit Ruth. Ruth may not be as bright as Tillie, but she certainly could prove that she can do much more than what her mother would expect from her. This proves that Beatrice does not really care about Ruth's failures either; all that Beatrice wants is to control the actions of Mr. Goodman, Tillie, and Ruth, altogether. Anything that may threaten her control is an immediate cause for rebellion on her part.
On a separate note, let us not forget that Paul Zindel was a Science teacher before becoming a playwright. This being said, imagine the number of incidents which, even current teachers, experience with parents that have "issues" which they choose to vent on the teacher and the system. Beatrice epitomizes the figure of the "problematic parent" who, inwardly, fears the power of their own children.