Election advertisements must carry a promoter's statement, which is missing from Mr English's advertisement.
In other cases the Electoral Commission has referred advertisements without the statement to police.
The advertisement also carried a parliamentary crest, denoting its funding came from Parliamentary Services.
That is against Parliament's rules on election advertising and Mr English could be forced to repay the cost.
Mr English has in the past been vocal about the use of Parliamentary Services funding, protesting against the ways Labour used the money during campaigns.
Late last night Mr English conceded there may be a problem.
"This advertisement was placed in a school newsletter in May, with Parliamentary Services approval. We endeavour to comply with all the rules - but in this case there appears to have been an oversight once the regulated election period began on 26 August'', said a spokesman for Mr English.
"The small cost will be included in election expenses as required and in future we'll ensure these advertisements comply with the rules.''
The Gore advertisement has also raised wider concerns about whether politicians should be advertising in school newsletters.
At a PPTA conference in Wellington today, Gore High School teacher Terry McNamara asked Education Minister Anne Tolley whether she thought political advertisements in newsletters were appropriate.
Mrs Tolley said she had previously advertised in school newsletters, and that for an electorate MP it was a good way to make themselves available to constituents.
"If it's just an availability - here I am, this is who I am, come and talk to me - then that's fine,'' Mrs Tolley said.
"Would I think that it's right to take a political stance in a newsletter? No I don't. School newsletters are usually information for parents about what's happening inside the school, it should be kept like that.''
The advertisement appears among a number of others below a banner that reads: "Thank you for supporting those who support our school''.
Mr McNamara told APNZ he had concerns about such advertising going out on school letterhead, and what message the school was sending.
"A school should not be backing, or be seen to back, or be paid for by, a political party,'' he said.
Schools Trustees Association president Lorraine Kerr said individual school boards made decisions about advertising but she could see a problem with schools getting involved in politics.
"Schools can't afford to align themselves to one political party because they don't know who their community, their families, represent or not,'' she said.
"Rather than get into that sort of battle they're better to remain neutral.''
