A private school teacher has admitted to having a sexual relationship with an underage boy, but denied claims that she sexually assaulted a second minor.

Sonia Mackay, 43, from Henley Beach, Adelaide, on Thursday pleaded guilty to one count of maintaining an unlawful sexual relationship with a boy.

The English teacher then pleaded not guilty to two aggravated counts of indecent assault, and one count of unlawful sexual intercourse with a second boy.

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Private school teacher Sonia Mackay (pictured, centre) has admitted to having a sexual relationship with an underage boy, but denied claims that she sexually assaulted a second boy

The second boy is said to be 'teetering on whether he wants to proceed with the charges', Adelaide Now reports.

Mackay sobbed while facing court in Adelaide on Thursday as both the prosecution and defence agreed the case should be postponed because the second boy's claims were a 'critical aspect' of the matter.

However, Magistrate Simon Smart said the court case should continue and the attitude of an alleged victim should not impede the judicial process.

'It may be an important aspect, but it is not a decisive one…I know from my personal experience, that prosecutors may choose to proceed anyway,' Mr Smart said.

Mackay was initially taken into custody by detectives from the Special Crimes Unit in December last year when she appeared in court in handcuffs.

After her arrest, a letter was sent home to parents informing them of her suspension.

Mackay was initially taken into custody by detectives from the Special Crimes Unit in December last year when she appeared in court in handcuffs

Mackay's name had been suppressed until her guilty plea, and the first boy's identity remains anonymous.

In South Australia, the age of sexual consent is 17, but this increases to 18 if one of the parties is in a position of authority over the other.

The prosecution alleged that in the counts relating to the second boy, Mackay abused her position of authority in committing the offences.

Mackay's defence lawyer Stephen Ey said that it was unfair for his client to enter pleas before the second boy's allegations had been finalised.

However, Mr Smart said that was not the point and that there had been plenty of time for prosecutors to speak to whomever they wished.

Mackay has been released on continuing bail to face the District Court in November.